Because the government is taking the opportunity to tax the ever living fuck out of weed, and everyone is just so happy to finally be allowed it that they don't care. That means that legally grown weed is super expensive, and usually restricted to only the few who are allowed to have permits. Naturally the permits are handed out through backroom deals. The little guy simply isn't allowed to legally grow it. So first they make the legal stuff really expensive, then they force those who want to grow it underground by not giving them permits. So if you can grow it there is a market, but you have to grow it as if it is still illegal because for the peasants it still is.