Why, breaks in BC of course.
I’m interested to know how you arrived at this claim. There’s numerous reasons people may not upgrade: 1) they may not see the need. They got their wordpress site working there’s no reason to ever touch it again. 2) Their OS may not provide an update to the latest version. I believe CentOS 6 only provides 5.3 for example, plesk is terrible at providing php updates. 3) Lots of people have an ‘if it aint broke don’t even touch it’ attitude so will just leave the server as-is until something breaks.
Not only that, those stats are somewhat flawed. As people have become more security concious they’ve stopped their servers reporting server-side technologies used. This will inflate the statistics for older, rarely maintained servers where the defaults are set to report the tech used.