It isn’t about the potential money to be made, it’s about the money being made right now keeping it illegal and people with that money already owning their own pet politicians. Sure the federal government will get a ton in taxes, and soon enough the marijuana industry will have a lobbying arm to rival alcohol, pharmaceutical, or prison lobbies . But they don’t yet. And right now they are up against those lobbies in trying to legalize it. On a nationwide level they simply don’t have the same amount of money to spend. And those established lobbyist have deep “relationships” with seated politicians. In other words they make consistent campaign contributions and expect certain results. The weed lobby probably doesn’t own too many politicians yet (although I’m sure they do and are working on it, anything that can make some money after all).
TLDR: it might be profitable over all to legalize it, but the people against legalization are making a lot of money with the current system. They don’t want to see the drug war soften or end.