Many good reasons, but before those, here's the thing about marijuana: marijuana does not kill you, it lowers your I.Q. level greatly, thus making you an idiot. So, whenever someone calls a marijuana smoker an idiot, they are right.
Now for the good reasons why marijuana is illegal:
1- it makes you an idiot. literally.
2- In the hands of minors and the immature, marijuana can create problems. For example, a group of friends could be camping out in the cold, and decide to smoke some marijuana. They'll get that 'high' for the time being, but, while in that state, they will have altered judgement and coordination skills. So, this group of people could, out of nowhere, decide to just play 'jack jumped over the candlestick' and jump over the fire. With an altered coordination level, one of those people could fall into that fire and burn alive. The friends would just sit there, watch him burn alive, and just laugh cause they have an altered level of judgement. Thus, instead of saving their friend's life, they just sit there and watch. (<- happened to a friend of mine. Luckily, one of the people there decided to NOT smoke.)
3. Making Marijuana legal for purchase, distribution, and smoking would create crime problems. One little fun act about marijuana: you can have black-outs. A black-out is basically a short period of time when you lose your memory and have absolutely no recollection of the events that had happened during the time of the black-out. For example, a man could kill his wife for whatever reason, and when the fingers are pointing in his direction, he could just claim that he was legally smoking marijuana, and that he had a black-out during the time of his wife's murder. Thus, he gets of the hook for man-slaughter. (<-kinda pulled outta ***, there).
"Do or do not, there is no try" -Yoda