The State has failed to cut off the illegal product. That’s the failure to honor the unwritten rule, the State kills the illegal product, just like bootleg liquor. The State only has itself to blame for the failure. The legal operations pay the Vig and receive nothing in return!
Black market marijuana is similar to bootleg liquor, but with one important distinction: producing liquor has a substantial cost (relatively speaking), while the cost of growing marijuana is so low that the barrier to entry in the market is effectively $0.
Weed is extremely easy to grow, so high prices and high taxes were never going to sustain themselves.