The barrier to medical studies has been the Federal Government (DEA) for several decades. They’re changing it but slowly.
Since 1968 the only place researchers could legally obtain cannabis for research was from a government lab in Mississippi, and as noted below, they wouldn’t even give you any. Researchers who did obtain it noted it was often old, low potency, and generally low quality, making it not very useful.

Linked DEA article:

On May 14, 2021, the Drug Enforcement Administration took an important step to increase opportunities for medical and scientific research. DEA is nearing the end of its review of certain marijuana grower applications, thereby allowing it to soon register additional entities authorized to produce marijuana for research purposes. Currently, the National Center for the Development of Natural Products at the University of Mississippi is the only approved supplier of marijuana for research purposes in the United States, and that production has been exclusively for the National Institute on Drug Abuse.

NPR notes:

Yet scientists still aren’t allowed to simply use the cannabis sold at state-licensed dispensaries for their clinical research because cannabis remains illegal under federal law.
“We’ll see a decade or more of explosive cannabis research and potential new therapies,” says Dr. Steve Groff, founder and chairman of Groff North America, one of three companies that has publicly announced it has preliminary approval from the federal government to cultivate cannabis for research.