Because in a lot of ways people would buy pot from dealers. Lots not a big money maker so the dealers would pressure on stepping up. It focuses more on the relationship that way. In the states that have legalized weed, it removes that dealer/pushing harder stuff and the harder drug use has gone down.
Since you've probably never bought drugs before. I'm gonna let you in a secret, drug dealers usually specialize. Weed guy or cocaine guy or Molly guy, etc. It's usually not a general drug store.
Also, the main gateway drug is alchohol for older people, as that was the first drug they used. The younger kids get their first experience with drugs from the myriad of pills parents cram down their throats throughout their young lives.