The same reason why there's never a "flush" in Congress and these folk hang on 15-20-30-40 years - 2 years at a time. The cost to compete against a person who can basically get all the media attention for free s/he wants, coupled with the "pork" in the way of deals, jobs and favors they can hand out, make it darned difficult to unseat a House incumbent.

Just look at the situation in Florida Congressional District Number 5. The incumbent (and recently re-elected) Hon. Ginny Brown-Waite gets her message to the voters a couple times a year via Congressionally-paid mailings. In addition, she can attend as an honored and sought guest at every business opening or event in the District, and the press follows her and there is a photo opportunity. In short, she is able to establish and maintain "brand" recognition, while any opponent needs to outspend at a ten-to-one ratio to get any comparable name recognition.

This is not to say or insituate that Ms. Brown-Waite is or has been a lousy congressperson. On the contrary, as her record shows a level of independence from the Administration in favor of her district versus her party. However, the honor of being a Congressperson (that's why they call them "Honorable") was never intended to be a career job, and one would like to think there are other persons in the District who can fulfill the role and responsibilities, and that a sufficient number of these "other persons" exist so as to be able to replace the incumbent after a term or two. If the position of congressperson becomes a "career," then it's no longer government "of the people," but rather government "by the clique."