I think Dr. Busse can probably answer this as well.
Very briefly, I'm thinking that it's because marijuana has only recently been legalized. In the past, the majority of research came from the FDA. They have really strict guidelines. All of the marijuana they use comes from one source. That source of marijuana probably comes from the 1960s in terms of the strains or in terms of the content. That's why a lot of the research....
It's also very difficult to get grants to support marijuana. The majority will be against it. If you look at side effects or substance abuse, you get grants more easily. That's why even the research itself is biased.