Here's part of the explanation from Democratic Congressman Joe Donnelly from Indiana:
I took many factors into consideration before ultimately deciding to vote against H.R. 1592. Not least of those factors was the input I received from my constituents. All told, I received nearly five times as many calls, emails and letters from opponents of the bill as I did from its supporters.
Christianists were organized, fired up by their hatred of gay people principled defense of federalism. Donnelly's subsequent argument is that he doesn't think hate crimes laws can abolish hate. Too right. So why does he support them for every other victim category?