Because not everyone who claims the title of "Christian" really IS one. Lots of folks may claim to be something, but their lives and actions reveal otherwise (for example: people who call themselves "Catholic" just because their families/ancestors were "Catholic").
I like to call them lower-case "christianists" who practice lower-case "christianism." They don and doff their faith like a smoking jacket, whenever it is convenient.
I have a feeling that support for Obama among REAL Christians is quite low.