In the course of my musings on Twitter, AmishDude suggested that the real motive here is that the GOP leadership is actually concerned about the implications of a landslide. Of all the suggestions put out there, this seems to make the most sense, and synthesizes the above theories reasonably well while addressing most of my pushbacks on them.
The idea is twofold. First, a landslide would present as much of a problem as it does an opportunity for those who might want to revisit the issue in 2015, especially if the GOP establishment (or its donors) believes this is a must-do before the 2016 elections. The base would be even more agitated after a big victory, and appalled at any compromise on this issue if the GOP picks it up in 2015. In addition, absent a majority, Democrats wouldn’t have the same incentive to support a bill that contained further compromises, especially since they already view the bill as a compromise in the first place. They’d be better off watching Republicans flail and fail to pass a bill as their own base abandons them; this is roughly what happened in the mid-2000s.
This makes sense of the timing issue. Perhaps the GOP really did plan on letting the issue die last summer, when taking the Senate looked like a 50-50 shot, and breaking even in the House seemed like the order of the day. But then the Obamacare rollout hit, and suddenly Republicans looked like they might enjoy a 2010 redux.