There isn't any super secret amazing reason Sessions is against Cannabis.
He is just very ignorant on the topic, and holds an ignorant and outdated opinion based on reefer madness propaganda. It's OK for people to be wrong, and it's OK to disagree with one of the good guys on what amounts to a tangential issue.
He seems principled enough to filter out his personal opinion on the issue and honor the will of the people and the laws that are on the books, so who cares if he's a little paranoid about the evil scourge of the devil's lettuce, a drug as dangerous as heroin.