The federal government of the United States has scheduled Marijuana as a schedule 1 drug suggesting that it has no viable medical use and is strictly an intoxicant. Repeated requests to study the medicinal purposes of the plant are repeatedly denied by the FDA, notwithstanding the fact that it's medical uses are beyond dispute.
I do not disagree or agree with marijuana trials, but I do understand the reasoning for why they do not occur. They do occur very rarely. However, our current laws and standards state that if there is a risk of harm for humans a study will not be approved. Since there it is assumed some damage or harm could occur from marijuana usage it is unlikely that approval for this research will come very soon.