Activists (and whatever politicians they were able to get on their side) have spent decades working on those issues and pushing them into the mainstream. People didn't wake up and just randomly decide that they supported marriage equality. Hell, it took them a long time just to get the big LGBT organizations on their side back in the 90's.

And the rigid party discipline might have been what passed the ACA in Congress, but making that a top priority for the Democratic party was because of the people who had been fighting for health care reform forever.

Same with the Civil Rights Act, the VRA, any sort of environmental legislation, etc. Progress never comes from the top down.