Take into account that alcohol comes in comparably big bottles and you need quite some of it to get drunk (if you are a professional of course). So, it is easier to control it's traffic than hunting for small pills which can be in everyones pockets.
Drug manufacturing isn't easy, most of the stuff is controlled and purchases of quantities of many required compounds will often immediately involve one or more government agencies, generally the DEA.
I see this a lot at work, I clear international freight through Customs (and any other applicable government agencies) for a living, and a lot of tariff numbers will flag as potential precursors by DEA.
