I read this is currently a big problem in California, most marijuana growers do not see the point in registering and doing it legally. It's just more paperwork and less profits. There's even less incentive now that the punishments have been reduced.
The myriad problems with just getting the political willpower to legalize drugs, is a starter. Then, there is the mechanism to do so - like what do you do with those already convicted? How does one make it so that it's even a realistic discussion being had by the average citizen? What about individual States? Etc...
Why would they pay more? First, convenience - it's right in the store, you can just buy it without having to do a shady deal where you might get killed.
Second, big drug companies at scale will be able to manufacture for much less.
Third, there is a premium you have to pay for anything illegal to compensate the seller for the risk. So they are paying higher than it's worth anyways.