The current Congress is a dismal failure and is desperate need of new ideas, procedures, and influence.
Political machines (local party voting infrastructure, redistricting power, media contacts, etc.) of incumbents make it very difficult to remove them from office.
Lobbyists and big-money campaign contributors usually direct their efforts at those in power, making it difficult for a new candidate to get off the ground.
Politicians are less likely to be focused on special interests and pork-barrel spending if they cannot stay in office indefinitely.
Politicians in their last term of office are more likely to ignore politics and media criticism to target what's best for the country, and they can work to establish tangible accomplishments that will build on their legacy.