Chait believes it's because political donors lean fiscally conservative and socially liberal:
[F]or Democrats, the donor class means you can't be too economically liberal, but donors do reward social liberalism. … The gay marriage vote is a reminder that the disproportionate influence of donors may be offense as a matter of principle, and its impact may very often be illiberal, but sometimes it has a positive effect, too. If Michelle Bachmann wins the 2012 election, the power of Republican donors will help restrain her rampant theocratic impulses.
John Sides provides other rationales.
The Dish 