I don't think Microsoft is stupid, they know that they have the lions share of developers- they're also aware that they are losing to linux in a big way when it comes to service deployment environments. I even heard Azure itself does not run Hyper-V on Windows anymore (though, who can be sure).
It's a smart play for them to have WSL- it means less friction to target Linux and will keep people writing software on Windows (which increases the likelihood of making software for Windows- especially developer tooling).
I think the parent is wrong that developing software on Windows is hard, my original comment up-thread was largely regarding the fact that in the late-90's and early-00's internet access was not common and getting your hands on a Microsoft Development environment was the furthest thing from easy. Linux, ironically, was easier as it came with all kinds of documentation and interpreters and compilers out of the box.