Jane McAlevey's postmortem on the always doomed Amazon organizing campaign also explains why the PRO Act can never ever ever be allowed to pass.
The conditions most workers in the United States endure when trying to form a union make the recent actions by Georgia’s legislature to institute further voter suppression seem tame. If the Senate passes the PRO Act, there’s no question the unionization rate would increase quickly, which is one reason winning its passage in the near future seems oddly distant. Despite the nation having the most pro-union president in nearly 100 years, the Senate remains immovable on issues far less challenging than major labor law reform; it wouldn’t even accept a federally mandated $15-an-hour minimum wage. And progressives have been trying to pass labor law since Jimmy Carter’s presidency—without success.
And, no, the answer isn't "because Joe Manchin." There are a ton of things the Democratic Party led by this so-called "most pro-union president in nearly 100 years" could do to rein that guy in. They choose not to, though. A strong labor law would "fundamentally change" the power relationships workers have with their bosses and boost the political power of the working class in general. But that's not what these Democrats were elected to do.