There is a risk of viewing things like this as directly causal
You do something you get an effect.
That is often over simplistic.
I think the key word, in sqad's reply is predisposition.
Pretty much all of us are walking around with predispositions to one sort of physical or mental disorder. We may go our whole lives and not develop it, or we might be exposed to some environmental factor like a drug, stress, polution a hormone change or any one or combination of thousands of possibilities and develop the condition.
What I'm getting at is that for some people cannabis may not increase their risk of developing mental disorders but for others it may increase it greatly.
Of course you don't know if you're in the risk catagory or not and when you're determining public policy you have to look at the population as a whole.
You also cannot reliably draw conclusions from the earlier decades in this way because there are so many other environmental differences between then and now - any number of which may have some degree of an effect
It is also one of those subjects where it's really hard to get a good understanding of the research because there ae so many apparently conflicting studies.