Part of this is because it's hard to sustain high levels of grass-roots energy over the long-term. And part of it is because the tea party no longer needs to fight the Republican Party from the outside: It can fight it from within.
When Rep. Tim Huelskamp (R-Kan.) first ran for Congress in 2010, he embraced the movement and earned its support (to the extent that the movement spoke with one voice). He emerged from a crowded GOP primary, beat his Democratic opponent by more than 50 points, and headed to Washington. There, he validated the movement's faith in his politics, repeatedly pushing back against the Republican establishment. In 2012, he lost his seat on the House Budget Committee as punishment from House Speaker John Boehner (R-Ohio).
Over the long term, the House Republican caucus has gotten consistently more conservative, as measured by DW-NOMINATE scores compiled by VoteView.com. The scores look at roll call votes centered on the role of the government in the economy; higher scores show more opposition to that overlap. Since the mid-1970s, Republican scores have consistently risen.
You can see the increase just since the administration of George H. W. Bush. The average Republican DW-NOMINATE score in the 101st House was 0.327. In the 112th, the Congress that Huelskamp entered? It was 0.707. Huelskamp scored a 0.996.
Or, put another way: Huelskamp's high partisanship/conservatism score was non-existent in the 101st Congress. By the 111th, the Congress during which the tea party emerged, seven House members had scores as conservative or more conservative. After the tea party sweep in 2010, that number doubled, constituting 6.2 percent of the Republican caucus.
This is why Boehner, after this week, will no longer be speaker of the House.
So why is Mitch McConnell still leading the Senate? That chamber also saw new Republican members elected in the tea party wave, including Sen. Mike Lee (R-Utah). Lee's score is a little lower than Huelskamp's, but it's been the highest in the Senate since he took office. With elections only once every six years, the Senate intentionally changes more slowly than the House. But it, too, has been changing -- and growing more conservative.
In other words, there's far less need for a tea party outside of Congress anymore. The Tim Huelskamps and Mike Lees of the world are doing their own agitation from within the Republican establishment itself -- an establishment that keeps moving in the tea party's direction.