There's a very complete System/360 through modern zSeries emulator [1] [2] that runs basically every IBM mainframe OS. IBM does not license their recent operating systems or languages to run on it. Though you can of course run Linux, or older versions of MVS or VM. Lack of OS and tool licensing is a showstopper for many users who might otherwise transition away from IBM hardware through emulation.
Are modern versions of IBM systems even popular compared to Linux? The reason these systems still exist is because they use legacy versions of software that could be covered by emulation. Any migration whatsoever would be contingent on the cost.
I'm sure z/OS is not nearly as popular as linux servers but there is a niche market for what it provides, just because it is old does not mean it is inferior.
[1] https://en.wikipedia.org/wiki/Hercules_(emulator)
[2] https://sdl-hercules-390.github.io/html/