Because the de facto war that already exist in the Atlantic, the next 'incident' is used by the US to casus beli to declare war to Germany.
Only difference to OTL going to be a few weeks if so much before the declaration and the side that declare war.
IMO the difference is more likely to be days than weeks. As I have noted before, FDR's December 9 radio address to the nation certainly sounds like a rehearsal for a proposal to declare war on Germany [1]--and it is possible that if FDR did not yet ask for a declaration at that time, it may have been because he was convinced from intelligence sources and decrypts that Germany would soon declare war on the US.
And as I have also noted, with regard to how hard a time FDR have had in getting a declaration of war through Congress, I think a Gallup poll from December 10, 1941 should settle that: " The December 10, 1941, Gallup/AIPO (American Institute of Public Opinion) poll asked. "Should President Roosevelt have asked Congress to declare war on Germany, as well as on Japan?": yes — 90%, no — 7%."
http://books.google.com/books?id=61WMf6XRVT8C&pg=PA209
The general reaction in the US press--including the former isolationist press--to the German DoW was incidentaly one of indifference. It was a mere formality, they said; the US and Germany were already really at war, Japan could not have pulled off Pearl Harbor without German inspiration, etc.
There is also incidentally no reason to think that if the US rather than Germany had declared war first it would have made any difference to the "Germany first" strategy which US planners had agreed on well before Pearl Harbor. As Louis Morton writes, by the summer of 1941,
"...the decision on the course the United States would follow in the event it was "compelled to resort to war" had, in effect, been made. The United States would make the main effort in the Atlantic and European area where the major enemy, Germany, was located, Just how the final blow would be delivered was not yet known, but the Americans expected it would require a large-scale ground offensive. In the Pacific and Far East, United States strategy would be defensive, with greatest emphasis on the area encompassed by the strategic triangle, Alaska-Hawaii-Panama. Implicit in this concept was acceptance of the loss of the Philippines, Wake, and Guam, Thus, in a period of less than three years, the Pacific orientation of U.S. strategy, developed over a period of many years, was completely reversed. By mid-1941, in response to the threat from Europe, the eyes of American strategists were focused on the Atlantic. It was there, they believed, that the war in which the United States was certain to be involved would be decided.
"These expectations were more than fulfilled. Though the war when it came opened with an attack in the Pacific, the President and his military advisers made it clear at the outset in the first of the wartime conferences with the British held at Washington in December 1941-January 1942 (ARCADIA) that they would stand by their decision to defeat Germany first. Not once during the course of the war was this decision successfully challenged."
http://www.history.army.mil/books/70-7_01.htm
***
[1] The course that Japan has followed for the past 10 years in Asia has paralleled the course of Hitler and Mussolini in Europe and Africa. Today, it has become far more than a parallel. It is collaboration so well calculated that all the continents of the world, and all the oceans, are now considered by the Axis strategists as one gigantic battlefield.
In 1931, Japan invaded Manchukuo without warning.
In 1935, Italy invaded Ethiopia without warning.
In 1938, Hitler occupied Austria without warning.
In 1939, Hitler invaded Czechoslovakia without warning.
Later in 1939, Hitler invaded Poland without warning.
In 1940, Hitler invaded Norway, Denmark, Holland, Belgium, and Luxembourg without warning.
In 1940, Italy attacked France and later Greece without warning.
In 1941, the Axis Powers attacked Jugoslavia and Greece and they dominated the Balkans without warning.
In 1941, Hitler invaded Russia without warning.
And now Japan has attacked Malaya and Thailand and the United States without warning.
It is all of one pattern...
Your Government knows that for weeks Germany has been telling Japan that if Japan did not attack the United States, Japan would not share in dividing the spoils with Germany when peace came. She was promised by Germany that if she came in she would receive the complete and perpetual control of the whole of the Pacific area and that means not only the Far East, not only all of the islands in the Pacific, but also a stranglehold on the west coast of North, Central, and South America.
We also know that Germany and Japan are conducting their military and naval operations in accordance with a joint plan. That plan considers all peoples and nations which are not helping the Axis Powers as common enemies of each and every one of the Axis Powers.
That is their simple and obvious grand strategy. That is why the American people must realize that it can be matched only with similar grand strategy. We must realize, for example, that Japanese successes against the United States in the Pacific are helpful to German operations in Libya; that any German success against the Caucasus is inevitably an assistance to Japan in her operations against the Dutch East Indies; that a German attack against Algiers or Morocco opens the way to a German attack against South America.
On the other side of the picture we must learn to know that guerrilla warfare against the Germans in Serbia helps us; that a successful Russian offensive against the Germans helps us; and that British successes on land or sea in any part of the world strengthen our hands.
Remember always that Germany and Italy, regardless of any formal declaration of war, consider themselves at war with the United States at this moment just as much as they consider themselves at war with Britain and Russia. And Germany puts all the other republics of the Americas into the category of enemies. The people of the hemisphere can be honored by that.
http://avalon.law.yale.edu/wwii/dec06.asp