The reason that pot is illegal at the federal level is that in 1970, after the Marijuana Tax Act of 1937 was repealed, the Controlled Substances Act in the 1970’s established Schedules for ranking substances according to their danger and potential for addiction.
Back then, they didn't know very much about whether or not pot was dangerous, they just basically knew that hippie freaks and blacks seemed to like it, so the drug warriors didn't. So in the absence of studies on the subject, they put cannabis in the most dangerous class (CLASS I) just in case.
SCHEDULE 1 (CLASS I) DRUGS are illegal because they have high abuse potential, no medical use, and severe safety concerns; for example, narcotics such as Heroin, LSD, and cocaine.
Today the medical uses of cannabis are well documented, and are used widely in lieu of opiates. That in and of itself is a legit reason they should re-examine the issue of whether or not cannabis belongs in Schedule I.
If they changed that, there would be no federal issue, and the issue would fall under the 10th Amendment,which states "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people."
Just like President Trump said on the subject.
NOTE: This is not and endorsement of cannabis, just a historical answer to your very good question.
Article ten is Null and Void for all practical purposes and has been since before the ink was dry on the Constitution. Even worse the Courts have upheld all sorts of violations and like it or not that unfortunately is Constitutional until it isn't.