The Power of No: Many Tea Partiers are people who hadn't run for office before 2010, or maybe had served briefly in a state legislature where they were bomb-throwers, not legislators. They won their primaries by promising to be the most conservative, Obama-hating member of Congress the folks of their district had ever seen. In contrast, almost none of the safe Democratic members got elected just by saying that they were the most liberal candidate in their race. Most of them worked their way up through the lower political ranks, getting used to cutting deals, making compromises, and solving problems for constituents. They may be very liberal ideologically, but they're also old-school pols in many ways.
That gives them a practicality that their conservative counterparts don't have. For instance, during the debate over the Affordable Care Act, most of those members of the Progressive Caucus preferred a single-payer system, but they didn't say they'd refuse to support reform unless they got that. They also fought for the inclusion of a public option, but when that got dropped too (just to appease a couple of conservative Democrats in the Senate), they didn't withhold their support for the bill over it. They could have killed Obamacare right then, but they decided to take half a loaf.