I was discussing Senator Sam Nunn with a Federal LEO I knew in Georgia and he told me Nunn would not run again because of some secrets which were known by a few too many people, and sure enough he was right. Nunn did not seek re-election.
I suspect that there is "something" on most of them.
Very few have the moral courage and backbone to stand up to blackmail.
On top of that, most of them engage in the behind-the-scene games, sort of like the nobles in "Braveheart".
So instead of having 535 independent men and women in Congress, you mostly have a bunch of puppets who are controlled by the people that really run things.