Lobbying. Cannabis is effective as a sleep aid, anti depressant/anxiety, pain medication, recreational drug, among many other things. If it were fully legal and people were aware of it as a substitute to many of the things I listed above, those industries would lose billions of dollars a year. So they have a huge incentive to kick some money toward politicians as endorsements/donations, so the politicians act on their behalf, which is shooting down any federal bill to fully legalize it.