Just as the title says, I'm really curious about this and I want to learn why LSW can't be a thing.
WSL1 was just a wrapper for syscalls. WSL2 is more of a virtual machine with tight integration into the host (
binfmt allowing you to call Windows binaries, filesystem integration...).
An integration of Windows into a GNU/Linux system in the sense of WSL2 would require you to be able to modify, compile and run the entirety of the proprietary, licensed NT kernel. The source code for this is not (legally) available.
So you'll be stuck to spinning up a Windows machine in a virtualization software of your choice for the forseeable future.
There have been Windows emulators like
wine for running Windows programs in the Linux environment. Also full virtual machine environments like
Virtualbox are available.
These aren't exact counterparts to WSL, but ways of running Windows programs on Unix/Linux computers have been around for a while.
The overlap between
- People who want "Windows being a subsystem for Linux, like WSL".
- People who have full access to closed source Windows (Microsoft developers), and could do it.
is the null set. There are no such people.
Secret (proprietary) knowledge of Microsoft Windows would be required, and Microsoft has no business reason to make access to Linux easy. And Microsoft has a Legal Department chock full of lawyers.
Obvious answer: because Microsoft haven't built that. Without Microsoft, nobody can legally modify MS Windows.
Running Windows that way would almost certainly require a customised kernel and Windows libraries to support the integration.
There's nothing stopping you running a Windows virtual machine under Linux, but it's the integration between the host and VM that makes it a "subsystem" in that sense.