The current Congress is a dismal failure and is in 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.
Lack of term limits leads to a system of seniority, meaning those who have spent the most time in office gain more power (in committees, procedures, etc.); consequently, politicians focus on staying in office, districts & states don't receive equal power in Congress, and fresh new elected officials have limited ability to make changes.
There is less chance for corruption of government officials if time in office is limited; new politicians are less likely to have the knowledge to exploit the system for personal gain.