iOS 10 seemed to be adopted a lot faster than iOS 11, like in January 2017 where it was sitting at over 70%, so unless we see a sudden uptick in iOS 11 installs, by all accounts it looks like iOS 11 is lagging behind. However if we had to venture a guess, chances are this might be due to the fact that iOS 11 seems to have had very buggy releases.
For example it seemed that the update caused quite a few issues with some very basic functions, such as with the calculator app, and also a couple of buggy instances with autocorrect. Users have also claimed that the update to iOS 11 has slowed their phones down, caused excessive battery drain, and more, which is why it is understandable that not many are rushing to update their operating system.
Hopefully this is something that will address in future updates, and maybe something they’ll pay more attention to in iOS 12 which should be launched in 2018.