It wasn’t within reach; republicans control the house; before midterms, the decisive vote in the senate was Manchin. Democrats introduce bills to legalize weed, but unless they get a big majority those are not passing, and a law from Congress is needed for legalization.
This is the best you can expect until more progressives are voted in.
Changing drug schedules, including removing a drug entirely from the schedules is a process that can be started by the DEA, HHS, public petition or Congress. Congress can just do it, while any of the others it involves DEA and HHS coordinating via the FDA and the DEA making the final call. IOW, literally the same process used to put pot on schedule III could have been used to deschedule pot entirely but they decided on schedule III instead.
This wasn't the act of the legislative branch, this was the act of agencies under the executive branch. Specifically the DEA and FDA which fall under DOJ and DHHS, respectively. Who in turn are headed by the Attorney General and Secretary of HHS, who are appointed by (and ultimately report to) the President.
When people claim that Biden could legalize pot, they aren't talking about something he has to negotiate with Congress and never have been - they've been talking about him ordering his direct appointees to push through the required bureaucratic process to do it themselves. And he eventually did, but only as a half measure.