My personal belief is very out there and not really too conventional, but I think it's worth pointing out.
People who have controlled this system in every aspect, from the creation of money, the creation of religion, to pyramid schemes like secret societies (yes I'm pulling the secret society card), know what effects these psychedelics do to the populous. Psychedelics, of course, educate each one of us and make us more spiritual. A more educated and more spiritual person isn't as likely to accept control, and is more independent, self-critical, and therefore, thinks more critically as a whole. Unless the drug doesn't negatively effect production (alcohol, caffeine, etc.), then it's banned.
An analogy would be how the black slaves in the 19th century were treated. Slave owners and the government itself banned blacks from education. Reading, writing, etc. were all prohibited because why? Knowledge is power. An educated labor force becomes aware of his surroundings and is less controllable, resulting in the top losing power. The 'slave owners' of today naturally ban psychedelics because of their truly educational characteristics. If people realized that we're all one and that love is eternal, there would be no war profiteering, Fox news, or a wealthy elite.
It all boils down to the powerful using their power to remain in power.