The reason why there aren't any bankers in prison is Enron.

Enron and Arthur Andersen were aggressively prosecuted. Many top execs went to prison, Fastow and Skilling. Lay would have gone but 'died' before sentencing. But Arthur Andersen also lost 85,000 jobs. Enron lost 4000+. Employees pensions were wiped out.

Politicians and prosecutors do not have the stomach for that. Citizens don't have the backbone to elect politicians who have that stomach. (US Attorneys are appointed by an elected politician and confirmed by elected politicians.)

This is really just one more wrinkle on too big to fail. That and regulatory capture. And also Citizens United. And a few other things.