Why? Because that's never really been done before and it would be interesting. There are hundreds of alternate takes on Superman, at least one of them might as well be gay.
I am not proposing that the New 52 Superman be made gay. Or that a Superman is created specifically to be the "Gay Superman." What I am saying is that D.C. is creating alternate Supermen all the time. The next time they create one of those, if it fits the story they want to tell, why not make that version gay while they're at it?