Definitely fragmentation: every vendor wants to pretend they’re Apple but they aren’t willing to invest in a serious team, plus the fact that nobody makes money after the initial sale means there’s no effort to ship improvements. Any iPhone will get half a decade of UI improvements but with Android you have to hope you get a model which sells well enough that the vendor & carrier actually ship anything more than critical security updates. The recent releases from Google aren’t bad but only a fraction of the market will see them without buying a new phone and then only if they buy the right phone.