The elephant in the room is that right wing politics intersects with a certain group of people that will never ever approve liberalization of cannabis (or any drug for that matter), and will lobby hard against it.

This is not only the case for USA - but something you see in other countries.

Maybe something more specific to the US, would be the people and corporations that benefit from keeping cannabis illegal, and thus lobby equally hard against it.

Sometimes market liberalization and conservative thinking does not go hand in hand.