A "Windows Subsystem" is a concept that dates back to the original Windows NT line of operating systems. Historically, there've been a number of supported "Windows Subsystems", essentially APIs for the kernel. In Windows NT 3.1, there were multiple subsystems: Win32, POSIX, and OS/2, plus a separate one specifically for security.

https://en.wikipedia.org/wiki/Microsoft_POSIX_subsystem

While WSL2 isn't implemented as an architectural sub-system (it uses a VM instead), WSL1 was far closer to the original architecture, providing a Linux compatible API for the Windows kernel.

I think it's because WSL refers to the Windows subsystem that allows you to run Linux, not to the Linux system itself. You still have to download and install Linux on top of it, or at least you did the last time I used it a few years ago.

It's hard to argue it's even a subsystem anymore. More like "Integrated Linux VM for Windows".

There may also be some trademark law precedent that forces this naming convention. Even on the google play store, if you have 3rd party apps for something, it's always "App for X", the name cannot be "X app".