I saw an article a couple months back that argued that the focus on marijuana was because other substances are less prevalent, so a war on meth or a war on heroin would take fewer resources.
I note on the ONDCP web site they show lifetime use of heroin for all age groups at 1.4%, meth at 0.3%, and 40.4% for marijuana.
So by their own admission, they're waging a "war" on at least 40.4% of us.