Schedule 1 drugs have not gone through rigorous and expensive ($billions) testing that allows them to be put into schedule II-V or unscheduled. DEA also refused to give out any schedule I testing permits that may have allowed certain drugs to be tested for medical use. However, certain organizations that were known for being anti-marijuana routinely received permits for their testing. The process also takes 5-10 years for drugs to be approved. I don’t think the policy changes occurred until the Obama administration. This is also why drugs like Psilocybin and LSD were not studied in the US. DEA wouldn't give out permits and no one had the money for it. MAPS and other organizations finally got the ball moving. Support from Johns Hopkins and other top tier research institutions provided some legitimacy to testing as well.