SCOTUS did not find gay marriage bans unconstitutional in that case. Only the 9th circuit did, and California intentionally stopped defending it at the 9th circuit because the 9th circuit is and was pro gay marriage.
Those two seem a little at odds. People are going to vote against it, but not when it's specifically on the ballot?
If 90% of party A supporters support the issue, and 70% of party B supporters support and issue and the election is close to 50/50 with B in power. B putting forward the issue can make them lose the next election because that 30% will either withhold their vote or vote for the other party.
But if that same issue is a ballot measure, then the 90% of A voters and 70% of B voters will overwhelmingly pass it.
This is what I mean by a motivating issue. Nobody will withhold their vote if MJ stays illegal. But there are certainly people (mostly religious) that absolutely will withhold their vote if a politician makes it legal. Even if that's a super popular move.
That's why pretty popular things aren't done. It's also why unpopular things can be easily done. If nobody withholds their vote because of the "send the kids to the mines" act (because they are happy about the mandatory Bible study), then a politician can get away with really horrible things so long as they make the core of their voters happy. After all, you aren't going to let the other guy win now are you.
It's what's broken about parties and FPTP elections.
Reverse nominal determinism