At this point in history there is no denying the medical benefits of marijuana. Compared to other illegal substances, marijuana is very safe to use even as a recreational drug. It's not highly addictive like opiates, and isn't involved in disorderly conduct, domestic violence or murder cases. Alcohol is legal and a majority of criminal cases listed above involve alcohol.

I have a few reasons I believe marijuana is not yet legal in many states for recreational purposes. First, marijuana usually slows cognitive responses. It can produce delayed response times. Perhaps the law for recreational marijuana should be treated as the laws for alcohol? The problem here is that someone under the influence of alcohol is easily detected. It's easy to smell on their breath and clothes. It's easy to see it in their eyes, and hear it in their speech. It's so easy to identify, within a few seconds your blood alcohol level can be reviled simply by blowing into a straw. The same is not true for marijuana. When or if accidents happen it's hard to tell if someone is under the influence. It's easy to conceal blood shot eyes and the smell on your clothing. Slurred speech isn't typically identified with marijuana use. As of now, there really isn't a sure fire way to detect marijuana use instantly or how high so to speak a person is.
Second, when drinking alcohol it can take a good amount of time to get intoxicated, drunk, or trashed. As a person is drinking the effects are gradual. The effects of marijuana are usually almost instant. For many people there really isn't a way for them to tell how hard the high is going to hit them. Of course the effects are different for everyone just as it is with alcohol, the effects of alcohol is more gradual.
Third, there hasn't been as big as an outcry as it took to get alcohol prohibition lifted. People who want marijuana legal need to push a little harder. Anyway, I might be ignorant just like others. These are just a few reasons I do not believe recreational marijuana use will not be legal any time soon. Feel free to educate me.