Why do so many countries allow the sale of a damaging drug like alcohol, but ban marijuana?

One of the reasons marijuana is banned in many countries is because its long term effects are unknown. Whether or not smoking marijuana puts you at greater risk of cancer and other diseases is not yet entirely clear, so many countries have taken action to ban it. We do know, however, that marijuana can impact brain formation when used by those under 21 whose brains are still developing, pushing some countries to press the ban further.