That may be a big part of the reason in the US, but Germany for example isn't big on decriminalization either, and we don't have anything similar to the US prison complex. I think a large part over here is that people think it's wrong and they don't believe in the individual's right to make a choice they consider wrong. Worldwide it's probably a mix of the two: powerful interests profiteering from the status quo and politicians and parts of the voting population not valuing individual freedom that much.