There is no political advantage to Government (I am talking Australian, but you can substitute US, UK French etc) in leglislating for same sex marriages - lobby for it is not powerful, and face it - unpopular, too many people don't want same sex marriages & are affronted by them. There is no money to be made or political advantage, so its not worth a Government risking being voted out of office by leglislating for the principle.
It is a result of living in a democracy I guess. But - attitudes are changing, people are becoming more considerate & aware (I hope)