Because it's not "not considered by anyone".
Democrats have been demonstrably decriminalizing and legalizing weed all over the country, and the Democrats in the federal government have been pushing and submitting and trying to make it happen.
It's republicans. They are the ones that continually stonewall a measure the vast majority of their constituents support, and they are the ones that somehow still get elected despite that.
Show me the democrats preventing legalization of weed federally.
Show me the democrats who invite cop associations to talk at their meetings about how dangerous weed is. Show me the democrats who are taking money from cop associations or prison lobbying organizations who very explicitly want to keep weed illegal.
Stop overgeneralizing! It's literally how things are this bad! Blame who is actually at fault!
Is it necessary to recount how this entire split is partially due to racism, anyway?
Many Republicans are just against it out right, and many Democrats are either indifferent or know that promising to legalize it will mobilize a subset of voters who prioritize it above else (or may just not vote at all otherwise, over 30% of Americans don't vote after all).
It'd explain why there's been so many opportunities to reschedule the drug, and why in some states even when they had the numbers to pass legalization, they still don't. Or do so with extra incentives (often the actual sale of it) to come later (vote next cycle!).
That's not the same thing as legalization as it's implemented elsewhere in the US.