Somehow they are adding RHEL, but not Fedora which what the majority of WSL users would want to use.
Fedora usership is significantly less than RHEL, and adding RHEL support to WSL helps the Windows Server use case
It's heavily used for enterprise and production use cases in organizations (often legacy type orgs though ik a F250 tech company that does this as well) who don't want to use additional spend building a dedicated Linux fleet and a dedicated Windows fleet.
Before you'd have to use Cygwin and deal with some level of non-reproducibility, but with WSL you can essentially get a MS supported Cygwin.
> but I would have thought Fedora was more common
The Linux workstation usecase is extreme niche compared to the dual server usecase.
I haven't touched Fedora in years.
At work we usually use a mix of Ubuntu, Red-Hat or the distribution of the cloud vendor.
In a different timeline I would be using Solaris, or NeXTSTEP (which I can get my fix with macOS).