It's simply a well known fact that weed is less bad for your health than alcohol, iess destructively addictive than alcohol (I mean: it's less likely that you'll develop a Lifetime Dependence to it) and a weed user is less dangerous to others than an alcohol one. I can source this.
Additionally a pretty big portion of weed is sold and consumed illegally. More than any other drug for sure.
If you made weed legal you could tax it, have stricter controls on quality (probably diminishing health damage) and also it might become harder for minors to get it. A legal shop owner is less likely to sell drugs to kids (illegally) than a drug dealer
Also: having to buy weed from a drug dealer makes it much easier to buy other illicit (and more dangerous) drugs. If one buys weed legally it will not be as easy for him to get into more heavy stuff