Most are looking to keep their jobs..that's why. Washington is all about doing favors....trading votes, buying votes etc. These new guys go in thinking they're going to just be able to vote...then they are strongarmed...it's that simple. That is one of the advantages of voting for those with Government expereince...they can roll with those punches and play hardball .....the young pups aren't seasoned enough to know 'HOW' to offset the "power brokers"....so they fold.
Term limits.