The legal market pushed the illegal market back. No one makes contact with illegal dealers to score weed. The shady weed dealers all went out of business. Ordering online or going into a shop and selecting 1 of 50 varieties with various effects made the entire process so much better than the illegal market pale by comparison.

That not only keeps kids from buying weed, but also it destroys the sales link to the illegal market that also sells harder drugs. This reduces the amount of people climbing the drugs ladder.