Colleges don't tend to conduct illegal research, and the DEA does not tend to grant waivers for research into marijuana.
In other words you do not have peer reviewed medical journal articles to support claims of significant medical benefits from the use of marijuana.
N.b. There are colleges and universities in other countries where the U.S. DEA does not have jurisdiction. There is, and has been for many years, ample opportunity and, I would argue, motivation to find solid evidence of medical benefits from the use of marijuana.