Too put it more simply ladies and gents', it's the doppelgänger of WINE.
That makes a big difference when it comes to the implementation of syscalls that have no remotely close analogue in regular Windows. fork() and exec() spring to mind - the hoops Cygwin had to jump through to fake those[0] are pretty eye-watering, but for WSL Microsoft added new kernel functionality to support them directly.
[0] https://cygwin.com/cygwin-ug-net/highlights.html#ov-hi-proce...