If you are saying that cannabis is a gateway drug, then you have to take into account that it is illegal. To get it, you have to make the decision to break the law and seek it out. The type of person that will be willing to break the law to get high is the same type of person who will be willing to get "higher" by using some other drug.
I occasionally used cannabis when I was younger. I was a National Merit Scholar and maintained a 4.0 gpa. After a while, I stopped using it because it was illegal and I did not wish to put myself at risk of criminal prosecution. I also stopped because I did not like using a substance that might cause damage to my body. For the same reason, I limit my smoking of cigars and drinking of alcohol.
I never tried any stronger drugs. I think that another part of it is the person taking the drug. Some people just can't stop, and continually try to get "higher".
J