I work with high school kids, a bunch of whom play D&D. None of them knew about the revised books.
It makes me wonder if the initial sales represent mostly long-term diehards - folks like members of this message board that follow the industry closely, versus "kids in the wild" that mostly just like to play but don't follow the industry much. But I'm also guessing that excitement over this from the diehards was a lot more tepid than it was for 2014, let alone 2000.
Presumably some of them will get it eventually - and I'd imagine holiday season will see quite a few sold.
I think another factor -- at least among diehards -- is that, as a revised version, it inhabits a nebulous zone between "reprint" and "new edition." People excited and/or curious for the latter are less likely to buy it. It doesn't feel quite like the "Event" of past edition releases, even similar ones like 3.5 (comparatively, it feels more like 5.3).
And as someone who isn't actively playing right now and would probably go a different route if I did start a campaign up (e.g. Shadowdark, Dragonbane, etc), I'm more curious about the DMG than I was the PHB.