Marijuana is a schedule 1 drug according to the DEA. That means it is considered to have no therapeutic use and a high potential for abuse and addiction. That is the most restricted category, even drugs like cocaine and fentanyl are not schedule 1 substances. Drugs in that group are the most restricted for scientists to study so in addition to all the other barriers keeping scientists from studying what they want there are a ton of government hoops to jump through. Marijuana was originally criminalized for culture war reasons and it is still a schedule 1 drug 100+ years later. There are also a bunch of fun laws like if you're a gun owner and you use marijuana, even in a state where it's legal, you're not allowed to own your guns anymore.