Because, as I stated in a previous response, it makes no such suggestion. The number of cocaine users who used marijuana first is irrelevant to the "gateway drug" theory. It means nothing. The relevant statistic here is the number of marijuana users who go on to use cocaine, and, as this and dozens of other independent studies show, THAT number isn't very kind to the WODdies point of view.
Let's make it tangible - we've got twenty people who have used marijuana. Eight of them have used cocaine. All eight of the cocaine users used marijuana first. At first glance, that would seem to make your point. But the number means nothing, because you've got twelve marijuana users who never used anything else. Now, the WODdies point to those eight users and scream "gateway drug", while the independent studies point to the twelve who never graduated to anything else and make the more accurate connection: a person who is predisposed to use harder drugs most certainly used or abused tobacco, alcohol, and marijuana first, because they had access to those things first. However, the statistical majority of marijuana users don't go on to other, harder drugs, and thus the "gateway drug" myth is debunked.
And, to play your "personal" game: I smoked cigarettes at the age of twelve, drank to intoxication at fourteen, and smoked marijuana, although not with any regularity, from the age of fifteen. At eighteen I became an abuser of the legal drugs Vicodin and Percocet, which I had easy, cheap access to. I never used any hard drugs, haven't smoked marijuana or tobacco in several years, and now only occasionally drink past the point of sobriety, and certainly take nothing stronger than Tylenol for pain.