Because mediation without the power to enforce it is worthless and once more without the demobilization of Russia/France Germany would put itself at risk. Who says this mediation result is accepted?
I will go once into more detail on this. In Germany the military thinkers and planers were aware that fighting a two-front war against Russia and France would be a hard nut to crack. For coming out ahead they came to the conclusion that it would be best to focus on one side first to close one front. For foreseeable reason they deduced that Russia would be far harder to beat down. Historic evidence and sheer size made Russia out to be a bitch to bring down. Therefore, France was the obvious choice. The plan to beat France quickly was the famous Schlieffen Plan, updated and changed by the Younger Moltke. This plan was seen and accepted as the best chance of winning a two-front war against these two Great Powers. Without it the chances for winning would be significantly lower.
On the other side France and Russia had one plan, to simply beat down Germany through a two-front assault. France with the clear intent to enter the Ruhr territory.
Problematic for Germany was that their plan hinges on many things. From the element of surprise, to speed to a multitude of other minor things. It was fragile in the area of flexibility. Therefore, any minor changes to the timetable or introduction of new elements stood to severely hamper its execution, which would severely hamper the chances of German victory in case of a war.
So Germany should put itself under existential threat in hope a mediation by a third power, not even a peer to either side will work out. I can see why that was not done. I think you measure with two sets of scale. If you put a gun to someone's head and then tell them to put their own gun away to negotiation while your own stays in place, you cannot then blame the one with the gun to his head for rejecting such an offer. That guy would want the gun gone from his head to do that, which is what Germany demanded.
Once more wrong, the German plans hinges on many elements and them not taking actions would play out detrimental to their mobilization plans while the plans of France and Russia accommodated such a time window. Therefore, one side is disadvantaged the other not. This is not hard to understand.
Let me state the first thing, an ultimatum is that an ultimatum.
Oxford Dictionary defines it so:" a threat in which a person or group of people are warned that if they do not do a particular thing, something unpleasant will happen to them. It is usually the last and most extreme in a series of actions taken to bring about a particular result".
To make it simple, if I give an ultimatum, you do not get to negotiate. The moment Serbia rejected the terms, and this was a rejection of them, they knew what that meant. The upper echelons of power knew what would come, they got strengthened in their resolve by the Russian ambassador and therefore went for it. Assured Russia had their back and with Russia, France.
Not through all of Europe and Wilhelm noted this on the side of the page. Essentially his first thoughts regarding the matter, considering how some historians attributed some form of pacifism to him and his general lack of political insight he is not the best person to support such a claim.
Such an interpretation is filled with more than a tinge of propaganda shining through. It ignores the tone and actions of Serbia beforehand. It ignores the celebrations held in Serbia in response to the murder of Ferdinand. It ignores the newspaper outright proclaiming Austria to be at fault for the murder, claiming A-H is using the murder to claim Serbian complicity for political reasons. Reducing the normal time of mourning as a slight. Honestly, they were massive dicks about it. I will give a small list and then maybe this bullshit will look differently. If you look at it in a vacuum, it is no wonder that you misinterpret it or come to such a quite frankly speaking loop-sided conclusion.
" On 29 June, Miroslav Spalajković, the Serbian minister in St Petersburg, issued statements to the Russian press justifying Bosnian agitation against Vienna and denouncing the Austrian measures against Serbian subjects suspected of involvement with irredentist groups. For years, Spalajković told the
Vecherneye Vremya, the political leadership in Vienna had been manufacturing anti-Austrian organizations, including ‘the so-called “Black Hand”, which is an invention’. There were no revolutionary organizations whatsoever in Serbia, he insisted. In an interview granted on the following day to
Novoye Vremya, the Serbian diplomat denied that the murderers had received their weapons from Belgrade, blamed the Jesuits for stirring up a feud between Croats and Serbs in Bosnia and warned that the arrest of prominent Serbs in Bosnia might even provoke a military assault by Serbia against the monarchy." -Sleepwalkers p.243/44
"Pašić, too, muddied the waters with ill-judged displays of bravado. In a speech held in New Serbia on 29 June, attended by several cabinet ministers, twenty-two members of the Skupština, numerous local functionaries and a delegation of Serbs from various regions of the Austro-Hungarian monarchy, Pašić warned that if the Austrians should attempt to exploit the ‘regrettable event’ politically against Serbia, the Serbs ‘would not hesitate to defend themselves and to fulfil their duty’. "Sleepwalkers.245
"On 3 July, for example, during an official requiem in Belgrade in memory of the archduke, Pašić assured the Austrian minister that Belgrade would treat this matter ‘as if it concerned one of their own rulers’. The words were doubtless well meant, but in a country with such a vibrant and recent history of regicide they were bound to strike his Austrian interlocutor as tasteless, if not macabre." -Sleepwalkers p.245
"On 30 June, the Austrian minister in Belgrade, Ritter von Storck, met with the secretary-general of the Serbian foreign ministry, Slavko Gruić, and enquired as to what the Serbian police had been doing to follow up the threads of the conspiracy which, it was well known, led into Serbian territory. Gruić retorted with striking (and possibly disingenuous) naivety that the police had done nothing whatsoever – did the Austrian government wish to request such an investigation? At this point Storck lost his temper and declared that he regarded it as an elementary duty on the part of the Belgrade police to investigate the matter to the best of their ability, whether Vienna requested it or not.
Yet, despite official assurances, the Serbian authorities never conducted an investigation proportionate to the gravity of the crime and the crisis to which it had given rise. At Gruić’s prompting, Interior Minister Protić did, to be sure, order Vasil Lazarević, chief of police in the Serbian capital, to look into the assassins’ links with the city. A week later, Lazarević closed his ‘investigation’ with a cheerful announcement to the effect that the assassination in Sarajevo had no connection whatsoever with the Serbian capital. No one by the name of ‘Ciganović’, the chief of police added, ‘existed or had ever existed’ in Belgrade.
66 When Storck solicited the assistance of the Serbian police and foreign ministry in locating a group of students suspected of planning a further assassination, he was provided with such a muddle of obfuscation and contradictory information that he concluded that the Serbian foreign ministry was incapable of operating as a trustworthy partner, despite the assurances of Nikola Pašić. There was no pre-emptive crackdown against the Black Hand; Apis remained in office; and Pašić’s tentative investigation of the border regiments involved in smuggling operations fell far short of what was needed." p.244
"Serbian official communications depicted Austrian recriminations as an utterly unprovoked assault on Serbia’s reputation, the appropriate response to which was haughty official silence." -Sleepwalkers p.245
"Spalajković’s widely reported claim to the press in St Petersburg that the Belgrade government had warned Vienna of the assassination plot in advance raised awkward questions – disregarded by the Russians – about Serbian foreknowledge." - Sleepwalkers p.250
Serbia had a long-standing history of provoking, disparaging and insulting A-H. Reading their response with this knowledge it reads differently. Considering how long these neutral observers would need any chance of getting this investigation on track would be gone. Suggesting that the Serbians would have no chance to hide any involvement after such a long time, is ludicrous.
In the context of their actions, their now known complicity in the assassination and the enmity between the two states, it was the logical conclusion of such a string of provocations. You cannot act this way against a Great Power without repercussions.