Now, who is surprised by this? Show of hands? ;)
A large segment of the public is not well connected with the black market for banned drugs. Or perhaps they were connected when in school or college but are no longer so after years in the workplace. One of these drugs becomes legal. The urge exists among the not well connected to try the formerly illegal substance. Assuming a market exists for a produce, ease of availability increases usage, whether it is marijuana or anything else.