That's because the government controlled price is set too high. At, say $400 per ounce of medical MJ, there's a lot to be had in the black market. At $20 a lid of ditch-weed, not so much.
In other words, a legalized commodity with an absurdly high price (for taxes) invites a profitable black market. It happened with moonshine, too.
The thing is, I can take the seeds from low-quality ditch weed, give them love and nutrients, and the result will be a very passable product, if not a good product.
The inferiority of mexi weed is not wholly genetic. It’s partly environmental. It is historically propagated by broadcasting the seeds by hand.