Not everyone is "cool enough" to find a street dealer who wont rip them off, and who will actually "plug them" with a good price.
Not everyone feels safe going to the street dealer
Not everyone is willing to get caught going to a street dealer
Some users are business professionals, lawyers, cops, etc and will not engage in illegal activities
Some people will buy the stuff legally just to say they did...
and most people who buy street stuff, will AT SOME POINT still end up going to the legal stores
AND, when it becomes recreation ally legal, and street dealers are then stealing from the government tax revenue... the law will shut them down with harsher sentences (as well as broader options to charge the dealers with; eg tax evasion, stealing state revenues, etc.), or they will raise their prices because it won't be worth it to sell so cheap.... ultimately driving the buyer right back to the store.