Because the U.S. up to that point had a long history of a) the Monroe Doctrine and b) demobilizing immediately after wars. Though we got dragged into WWI, we even demobilized right after it and started referring to that war as "the war to end all wars." War with Germany was a super unpopular idea in the U.S. I recommend William Manchester's book The Glory and the Dream for the background. Though FDR was in fact trying to build that support (and, sure, I'll listen to the theory that he allowed had the intel but let Pearl Harbor happen) he hadn't really gotten there. Hitler declaring war, an act no one but himself made him do, gave all the PR needed to get support for the two-front war.

Look at the actual history of U.S. in protracted wars, such as the Civil War (ie from the North's perspective) or Vietnam. The public gets sick of fighting them. Also, imagine the kind of warfare the U.S. had to do against Japan -- island to island, against suicide runs. In this timeline, we might have grinded through more citizens but not had the A-bomb in time to speed up the surrender of the mainland.

In the meantime, if Hitler wins the Soviet front, he has all the oil he needs. Or, if he wins the U.K. front, he can set up a much better defense against U.S. invasion.