Look at pornography as an example. Its much more of an issue now because its so damn accessible then it was 30 years ago. When something is easily accessbile more people will try it.
That should be a pretty easy thing to understand.
If you don't think marijuana is easily accessible you're up in the night. It's easily accessible to EVERYONE and it has been illegal for decades. Legalize it and it will cost a little bit more and kids will have a significantly harder time getting it.
I'm sure there are some people who don't use marijuana because it's illegal, but I think most people who don't use it now won't start just because it's legal. There may be some people who try it fr the novelty when it'e legalized, luckily it is not addictive like cigarettes and alcohol.