Familiarity is all it is. There are countless tales of Android users who couldn't wait to get back once they switched to iOS. For sure iOS does some things 'better', but then so too does Android. If someone uses iOS for a few years and then switches to Android, it's completely understandable (and obvious) that they'll pine for how things used to be. The acid test would be to use Android for three years and then to decide. Otherwise it's not a level playing field.
Or it could just be that one works better for someone's needs.
Familiarity definitely plays a role, but to dismiss any desire to go back as "oh its just because you're used to the other" doesn't quite fly.
I use Android and iOS side-by-side. There are things I prefer about iOS - yes its more familiar to me, but it also just feels more polished. And of course, a big reason simply is my friends and family are on iOS devices making it easier to communicate and share with them.
It's just about what works best for someone.