Probably because marijuana use damages young brains. Potent strains, which are now commonplace -- if not the norm, cause psychosis. People end up in emergency rooms, especially with edibles. Long term users suffer from brain damage, mental, and behavioral disorders. It's perceived as harmless, and thus is easier to accept as a supposedly innocent long term habit, when the truth is that it silently robs long term users of their motivation and destroys dreams. People become enamored with the altered state of consciousness early on, to the point where it permanently interferes with their life path.

Marijuana is still strictly disallowed in the military, for clearance holders, and for employees of government agencies such as the FBI for good reason. It impairs judgment.

While I have personally used the substance in the past, I do applaud the current administration for banning anyone who has ever tried it from working in the White House. Many agencies have reasonable policies for past use. I believe the NSA allows for past use no more recently than 12 months, and the FBI for 36 months.