Because the middle class owns the majority of the wealth both parties need to give away to get elected. And middle class voters seldom punish politicians who act against their economic interests, because they let the MSM get them worked-up about symbolic issues and whatever the distraction-of-the-year happens to be.
The Tea Party's focus was supposed to be the driving of statists from power, regardless of their (meaningless) stated party affiliations. Many Tea Party-supported politicians, upon gaining office, immediately became distracted or coerced or bought-off and took their eyes off of their mission to shrink government, allowing the Alinskyites to successfully caricature the Tea Party as a whole as a group of wild-eyed social reactionaries (find a college student anywhere today who doesn't believe that).
An true opposition party in these times has one mission, and one mission only: reduce the power of the state. Sunsetting laws, eliminating agencies, slashing entitlements - these are the only things that matter when the middle class looks for a politician to support.