No particular story about legislative malfeasance affects the public in any demonstrable way, but the constant drumbeat of stories adds up. Secondly, you've got the politicians reinforcing what the media says. And the challenger is calling the incumbent the same thing. Both of them are beating on the institution. It's the system that doesn't work.
Legislators pander to what they think the public wants to hear.
All the public encounters is conversation about how bad it is. Indeed there are examples of corruption in legislatures. Some of the scandals are real, they're not imagined. And people can only generalize from the worst possible cases to all cases. An individual representative may be well-regarded, but he or she is different from all the rest.