Before I explain the circumstances why France isn't invaded I must admit, that this Tl could includes ASB because i make a roll every year for every Great power like in nation games and i try to make realistic events, but there are still things that you would call ASB.
So for Prussia:
Prussia fought a war with Austria, to gain more influence in Germany, in 1794. On her side Prussia had Monarchist France, The Netherlands ( Which control Indonesia and South India ,the Falkland Islands, South Africa and some minor colonies in Fire Land.) , The US and Venice, while on the other side there was Russia, Poland, Austria, Denmark and a weakened Great Britain( had a short but bloody Rebellion in Scotland, which failed)
In 1798 they won but for a high price. Although this, they were able to form an much earlier but not so stable German Reich, which saw already two Bavarian revolts. So they are not able to invade France yet.
For Spain: Spain is in a struggle with Austria for North-East Italy (
Queen Maria I gain the Throne of Naples, Sicily and Spain in 1800 and is able( by have a good roll for Naples) to unite Italy, which claimed North Italy from Austria.
Great Britain: The UK has lost his name as world power 1 after a American Independence War, that involved more European Powers, that even invaded Great Britain it self. They didn't lost many colonies , but the OTL USA, but the Trust in the King was weakened and a Scottish and republican Revolt happened. Now they are still fearing revolts and don't dare to leave there home isles