From what I've read, first of all, it has been used as a recreational drug, so maybe it is considered a controlled substance and docs want to keep it away from people's homes.

Also, it is a powerful drug, sometimes used to help maintain the patient's anesthesia during surgery. It works quickly for most people. So I guess it's a fast way to calm patients down. And probably dangerous in the wrong hands for that reason, too.

Those are two ideas I get from reading about it, anyway.