I think alcohol is the most abused substance because a lot of who use alcohol go insane and out of control. Alcohol can also cause dangerous accidents which is very bad.
Alcohol gives the user a temporary high. People may drink for sociability, fun, or ritualistic reasons. These could spiral into power, escaping problems, or abuse. Alcohol is also pretty addictive, and it's very hard to stop if you gain a reliance on it.
