Because, NOM President Brian Brown says in the Wall Street Journal, the President knows it will lose him more supporters than it would gain:
The political pressure for Mr. Obama to endorse gay marriage doesn't seem to be there at the moment. The Human Rights Campaign, a group that lobbies on behalf of of gay people, has already endorsed Mr. Obama's re-election. "There's no doubt that we would prefer the president to support and embrace marriage equality sooner rather than later but he's going to do so at the time when he feels right to do so," said the group's spokesman, Fred Sainz.
Mr. Obama's opponents are prepared for the moment, should it arise. "The gay issue is going to be big primarily because there's going to be at least one ballot initiative," said Brian Brown, president of the National Organization for Marriage, which opposes same-sex marriage. "If he comes out in support of same sex marriage I think that clearly would undercut his chances at re-election."