Of course, they may not have a choice. Republicans have made repeal a top priority, so the fight is going to take place whether or not Democrats want it. But I think it also reflects a changing dynamic, as well. According to Greg Sargent, who was among the first to spot the shift, House Democrats in particular are aware of the opportunity they missed last year, while the party and most of its leaders were trying so hard to save endangered incumbents.
When will Democrats learn that running as a Democrat who hates Democrats doesn't really work all that well. There's another option available to voters, the person who hates Democrats who isn't actually a Democrat.
I get that Dems here and there might buck their party on major issues, but that's entirely different than the entire party keeping quiet about what was, for better or for worse, their signature accomplishment.