Java doesn't run on iOS. Period. In fact it's almost impossible to build an iOS app using Java - the only ways to do it are essentially to transpile Java to some other language, so Java isn't running on iOS at all.
iOS is one of the most popular operating system in the world by any measure. It supports a lot of languages.
But that is not Javas problem, but IOS. They could use available languages, but they refuse and instead make you use their own programming language, swift. Even more vendor lock-in. Once you start using Apple shit, you join a cult of technology dictatorship.
By "most popular" you mean most used, right? I'm asking because the word "popular" sounds like people would actually like the OS. I am using android solely because of the lack of alternatives (iOS being even worse). In general, mobile OSes are complete horseshit if you ask me.
You know what works on all operating systems? Writing a makefile that compiles for all operating systems (unless you are using windows for development but then you are too far gone from the path of truth)