Part of the reason you see less research on marijuana is because there are more obstacles to doing such research. Here is a 2017 National Academy of Sciences report on the topic:
> [...] the federal government has not legalized cannabis and continues to enforce restrictive policies and regulations on research into the health harms or benefits of cannabis products that are available to consumers in a majority of states
FFS, if I would want to invest into something that has barriers to conduct a research, I'd give zero fucks about the law. Zero. These governments drive me nuts.