... because they didn't.
...and when they do, it becomes because they don't have a large enough majority. If they had a filibuster -peoof majority, it would be because they don't have a veto-proof majority even if they also held the executive.
To put it another way, Biden had the power to fully legalize marijuana at any time because drug scheduled can be changed by process from either the legislative or executive branches. Instead he had it rescheduled. Because Dems only operate in half measures federally, because they are controlled opposition.
You're right, it's currently still hung up in procedure. Even then, he could have ordered his DWA to reschedule it, and instead had them do what they've done. He also waited long enough that when it happens, Trump will get the credit despite having had nothing to do with the process.