What prevented investors from discovering that Bernie Madoff was perpetrating a financial fraud on them?

Madoff hired an agreeable accountant who issued unqualified opinions about investment values without verification.

Madoff had two funds, one for clients and one his own. If the returns in his clients' funds dipped he compensated them from his own funds.

He bribed regulators so that they would not investigate him in the face of warnings from other investment industry peers.

He covered up investment losses by using contributions belonging to one group of investors and distributing them as return on investment to another group of investors.