I think the illegality of marijuana is what causes it to become a "stepping stone". You need access to the drug, so you need (or needed, 20 years ago) to find someone to sell it to you. This person was the drug dealer, or a customer of one. The drug dealer had a little of everything -- merchandising = profit -- so if you heard about another drug, like cocaine or crack, you could ask about that, and get it.
I think it's different today, because medical mj doesn't connect you with the drug underground.
On the other hand, doctors overprescribing vicodin, a pretty addictive drug, has become the gateway to heroin. It's because the drugs are similar.