I think Gingerbread had the biggest impact, didn't it?
the color themes of new version beyond 4.4 are just so cheesy. updated then down-graded
having used Android at versions 2.1, 2.3, 4.1 and onwards until 5.0, frankly after 4.1 there was really not that much of a "need" to upgrade. Never seen any app that won't run on 4.1 (heck many still run on any version down to 2.3, if not even 1.6), and most utterly important upgrades came around that time. The hardware itself will die before you feel like the software needs change, and that's probably what's killing the older versions, I bet very few people actually check the version the phone/tablet is running before purchase, it's mostly about companies pushing the update.
Good part of that "lack of need" feel could be because of OEM modifications of course, but that still counts.