I really think it comes down to the fact that there is no way to quantitatively determine how high a person might be at any given time. With alcohol, you can get a finite measurement of blood alcohol content. Considering that marijuana stays in your system for much longer than alcohol, it would be difficult to impose laws based upon saturation thresholds as is done with alcohol. In addition to that, differing strains/growing conditions of marijuana provide a different level and quality of intoxication which cannot be quantitatively measured. On top of all of that, people have vastly differing tolerances to the mighty herb. In short, if you can't measure the effects, there is no way to prosecute. Law makers want to be able to prosecute for driving under the influence, public intoxication, etc. and they effectively can't do that. That's why it is still illegal...