I don’t think they have been bought off at all. I think it’s a deeply rooted cultural prejudice that has transcended party lines. It is often much more prevalent in older politicians, though not exclusively, and it seems to be based on an equation of marijuana with social disruption.
It’s bunk, of course. Alcohol is far far worse. Of course, we realized after Prohibition that, well, prohibition was dumb, but because it has been easy to associate weed with people who were not part of the “real” (read, white, Protestant, hetero, right-wing, capitalist) America, and because everyone has to have someone to look down on, marijuana became the near-perfect whipping boy, sort of like waving the bloody shirt after the Civil War for GOP politicians.
At this point, to take marijuana off Schedule 1 and in effect pave the way for national legalization would require so many people in power to backtrack on deeply held cultural beliefs that it’s highly unlikely to happen in this generation.