It's not just or even mostly the direct "kid smoked weed went to jail". It's growing up in a culture of drug war were the police are the enemy cause they round up, hassle, and jail your older brothers, parents, everyone you know for doing things you know are not criminal. The pushers, gangs, lookouts, mules, etc. Surround kids with crimes and criminals, they will pick up the lifestyle.

Pot is not a gateway drug. But pot illegalization was a gateway into crime.