Because threads like that (marriage equality) usually lead to trouble... and by trouble, I mean people with two very opposing ideologies fighting it out in a thread. The argument about whether it should or shouldn't be allowed doesn't really matter in the end because the ultimate choice is up to SE. I can guarantee that ANY argument that has come up in those threads has been said before... in the real arguments about real people fighting for marriage equality in real life. (Don't get me wrong... I'm all for it, but I know why those threads can get toxic real fast.)

Also, keep in mind that even asking why a thread is deleted (whether it's specific or general) might lead in this thread being locked and/or deleted.