Windows NT had a fairly complete POSIX subsystem (for the time) [0] (but not virtualization layer) so you’d only be able to use Linux if it could compile as a rump kernel (it can’t).
That was followed by a replacement subsystem, Windows Services for Unix [1], which later used a “real” Unix compatible layer after their purchase of Interix.
WSLv1 was therefore the fourth subsystem available (though it couldn’t do some of what the previous ones did, and did some of what they couldn’t), and the complete rewrite as WSLv2 is at least the fifth such attempt.
