Several reasons. The economy is a catastrophe, and Presidents alway lose when the economy is that bad.
Obama is 6'1" and Romney is 6'2". The taller guy always wins.
90% of blacks will vote for Obama no matter what. Hispanics won't. One reason is that Romney has Mexican ancestry.
Another is that Hispanics hate blacks (I lived in Albuquerque for a while).
Another big reason, and it shows how stupid Jesse Jackson and Al Sharpton are, is George "the White Hispanic" Zimmerman. He ain't exactly "Hispanic," being half-Jewish and half-Peruvian, but he looks Hispanic. And the attacks on him by the race pimps is going to drive more Hispanics away from the Democrats -- the party of blacks as it is.
As for the whites who voted for Obama, most have learned their lesson now they have realized Mr. Hope and Change is a continuation of George Bush.
Then there is the First Wookie. Boy did she screw up with her mouth, her attitude and her ugliness.
These reasons are why Obama will lose.