No, mostly likely because there are 250 million more devices than there were the last time this was measured.
Every year Macrumors writes stories like this, with absolutely no sense of context. As the installed base of devices grows the percentage of devices that upgrade quickly (or at all) will decrease. It's simple math. Macrumors tries to paint it as there being a "reason" for it, like a conscious reason. There isn't. It's just what happens when the installed base grows, and this particular one grows by almost unimaginable numbers every year.