Getting permission to do research on a schedule 1 drug is pretty tricky, from what I understand? Since marijuana is schedule 1 it's termed as having a) no medical benefit and b) a high potential for abuse, which puts you in something of a catch-22 situation: you can't justify the research on medical grounds because the drug has no medical benefit (or it wouldn't be sched 1), and you can't justify research for exploratory purposes because of the potential for abuse (or else it wouldn't be sched 1), and then it becomes hard to find research that says "Actually, weed has medical uses and isn't nearly as abusable as heroin" because no research can be conducted.
Edit: To clarify, that's not to say that marijuana _should_ be schedule 1, only that it _is_ schedule 1 and moving something out of sched 1 is challenging because the research position is tricky.