Sad to say, the most important special interest group you didn't name is the stupid voters. Whatever secret ambitions or fears cause them to do it, they keep electing whoever most convincingly promises to keep the gravy train on schedule.
True, though sometimes there aren’t viable options, and that’s by design. Too many figure their incumbent is A-OK and the others are lousy, but I guarantee you that they are not A-OK and neither is their opposition party challenger a good deal of the time. Even if you do get a “good one”, it doesn’t take long before they’re assimilated into the hive. My rule of thumb is that 6 years is all it takes for the good ones to become a part of the problem, sometimes less.