I have no idea of the relative numbers, but I can see two - maybe 3 - reasons for starting to get edition-insistent with 4e.I get that some people really like 4e over all other game systems and won't switch, but I just can't imagine that the majority of the community is that way.
One is obvious enough: you find it to be a better game. Sure, 3e was great, at the time, but 4e is better, you don't want to go back. For that matter, AD&D was fun for a long while, but once you had 3e, why go back? 5e doesn't need to /be/ 4e to satisfy this set, it needs to convince them it's /better/ than 4e. As they've gone from one ed to another, clearly, in the past they've found each ed better than the last, so past performance is on WotC's side with them.
Another is the same reason so many are so insistent on 3.5 or 1e or 2e - because they started with it and it defined the game for them. IMPX, there are a lot more new-to-D&D players of 4e than there were under 3e. I don't think it attracted particularly more (particularly as it had less than half the time to do it), but I get the feeling it retained a lot more of them. Suitability for casual play and new-player introduction had something to do with that, as did the relative ease of DMing. In any case, there's not a lot to be done about it, power of first impressions and all.
Finally, of course, there's just getting in the habit of pushing back against the edition war. 4e was embattled from the start, and that inspires an attitude.