Maddow brought this issue up, and it's something I've pondered since Prop 8 in California.
Personally I think it's because the bigots are more passionate about their position than your *average* person who supports marriage equality. I think many, perhaps most supporters of marriage equality are somewhat casual about it. I live in NC but I'm registered to vote in CA, so I couldn't vote on the recent measure.
The Republicans know full well the shift taking place in public opinion. They're gettin' while the gettin's good. These state-by-state constitutional amendments are a preemptive measure to make change take place more slowly.