That organized crime already existed and this is just one of a dozen or so income streams for them. Hard drugs, human trafficking, and money laundering are probably orders of magnitude more profitable.

Also: people get murdered all the time, despite it being illegal. Clearly the law isn't working, so let's make murder legal! /s

Oh, and legalizing weed didn't do jack shit about dealers selling weed; the dealers are much cheaper, provide better service, and often better product (well, except the illicit stuff is likely to have much higher pesticide content.)