My impression is that in part he waited this long because he initially was against it. He's stated from the outset that he's not in favor of drug legalization because of what's happened with his own sons. But presidents, like the rest of us, can be influenced and come to change their minds over time. And it took him a couple of years to get to this point.

But my cynical half says that the other part of why he waited until now is because he actually wanted to wait longer, and use the supposed electoral bump for his own 2024 re-election campaign, but that someone convinced him that if he couldn't secure a favorable Congress now in 2022, that waiting until 2024 might prove to be a moot issue.