Too bad the congressmen he blackmailed didn't know he was gay. At the time, that would have been powerful information.

It's important to understand that the FBI's power was very asymmetric.

J Edna Hoover could assign a team to gather kompromat on a person and to surveil them or harass them.

A Congressperson has no equivalent ability to threaten the Director of the FBI. At best, they could spread a rumor, but a rumor doesn't catch fire when journalists / editors had too high of ethics to repeat unverified rumors.