The Banks get in trouble, and Gov has to step in. So Gov, reasonably, add regulations and restrictions. But the law can't be really specific, it requires gov employees to actually examine the bank and make decisions (eg about risk levels, etc).
The banks have a really large incentive to chip away at the effectiveness of the regulation. They hire lots of lawyers, consultants, notable economists, etc and just keep pushing on these rank and file gov regulators. They buy influence with politicians, and use that to pressure the regulators. They hire some of the regulators at very high pay, sending a signal to the others: play ball and a nice job awaits you.
Over time, they just wear down the regulators. The rules are interpreted to be mostly ineffective and nonsensical. Often at that point the politicians come in and just de-regulate.
The banks just have the incentive and focus to keep at it every day for years. No one else with power is paying attention.