As one conservative House aide put it, “We can’t score touchdowns on every play; our job is to put points on the board. But all they want us to do is throw Hail Marys.”
There’s probably a more basic, personal level at work here. People who got into politics because they believed in conservative ideals are probably somewhere between severely annoyed and totally flabbergasted at having to constantly prove their conservative credentials, no matter how many times they (for example) vote to repeal Obamacare.
Granted, we’re a long way from a total crack-up here, and even longer, probably, from most mainstream conservatives in the House choosing to be legislators rather than accepting the role of props for these outside groups. And Alberta correctly points out that there are mixed incentives here: Heritage-funded junkets probably make the job a lot more enjoyable than it would be otherwise, and many members must realize that Heritage and other outside groups are a good landing point once they leave office, voluntarily or not (incentives that Fenno did not include in his understanding of the House).
But perhaps it’s a start.