Because of RINOs like McCain. McCain gave some lame excuse for not supporting a constitutional amendment because he thought DOMA was adequate when any idiot could of forseen Federal courts striking it down and the case going to the SCOTUS with Kennedy will be providing the fifth vote to strike it down.
MARRIAGE VOTE HIGHLIGHTS LIBERAL HYPOCRISY
http://www.donfeder.com/articles/0410marryVote.pdf
OPPOSITION TO MARRIAGE AMENDMENT
BASED ON HYPOCRISY AND CYNICISM
http://www.donfeder.com/articles/0606MPAvote.pdf