The number suggest either:
(a) Android users are satisfied with KitKat and scared of updating to Lollipop because of fears of Google and/or bugs.
(b) Android users have gotten frustrated with the Android experience and stopped using their devices before Lollipop.
More like they're at the mercy of handset vendors and carriers that don't want to spend the time and money to configure, test and roll out Lollipop for their installed base because they're more interested in selling them new phones. That's one reason why I bought a Nexus directly from Google.