My understanding is very few schools had a plan for making iPads effective classroom devices when they bought them. So for many, the attachment was zero.

But the other thing is, I don't think you're seeing a "disruption" of an existing market. In 2009 and before, nobody would ever have conceived of the insanity of buying one computer for every student. I think this is closer to a new market than anything else. Because neither iPads nor Chromebooks really infringe on the sorts of purchases most schools' IT purchases were prior: Computer labs and office PCs, which are likely predominantly still Windows. (And oddly excluded from these numbers.)