1. To paraphrase the previous comment, many defendants plead guilty in order to pass for "easy" and avoid the hard time that "hard cases" get. ("Admission" to the drug court program is a better deal than prison-- you are better protected against gang-rape, persecution etc.) That in turn enables prohibitionists to "prove" marijuana is a problem-causing "drug" by pointing to the large statistical number of marijuana "offenders" admitted to the drug court program.
2. Both in prison and in "drug programs" are to be found well-paid bureaucrat "therapists" punitively pretending to "treat" the marijuana "drug offenders" on the premise that cannabis must be classified as a "drug" rather than as an "herb" or "nutritional supplement" which would place it in a non-punitive category.
3. Police, prosecutors, judges, bailiffs, guards are all benefiting from their share of over $30-bil./yr/US nicotine $igarette tax money, and for that they reward the big tobackgo corporations by suppressing cannabis as an illegal "drug" and thereby suppressing the anti-overdose smoking and vaporizing equipment (which, if cannabis were legal, could become readily available to tobacco users and replace the now entrenched profitable hot burning overdose $igarette marketing format), on the premise that such equipment is related to illegal cannabis and thus also illegal or "evidence". (How would you like your $600 Volcano vaporizer to be confiscated, or used as evidence to convict you of a crime?)
4. Big pHARMa doesn't want unprofitable homegrown cannabis to get a chance to (a) outclass their currently profitable proprietary Stop Smoking remedies at $20, $40, $60 a box and a 10% success rate, and (b) cause an abrupt decline in cigarette smoking, leading to a consequent decline in sales of profitable Statins and other drugs now used to manage chronic tobacco-related degenerative diseases.