I don't know. They had to start by striking down the federal benefits part of DoMA, and then return the issue to the lower courts to build up sufficient decisions (and precedent.) It takes time to do this. Frankly I'm astonished (and proud) that public opinion has shifted so quickly.
It's all the people coming over the last 15 years. It's easier to hate an abstract notion of a person--all those gay stereotypes, plus what you hear in church--than someone you know and care about.
