@hammarbtyp: "The questions is why?"
The reason is that the Achilles heel of Windows when it comes to portability is that the only reason people buy Windows is to run proprietary x86 applications. Microsoft can port Windows, MS Office, MS SQL Server, Dot Net, and various other bits of software that they own to ARM, but each customer will always a handful of third party applications that haven't been ported and that they don't want to do without. Being able to run those poorly is better than not being able to run them at all.
It's a chicken and egg problem Third party developers won't support Windows on ARM until there's a market for it, but customers won't create a market for it until there's enough third party software to run on it.
There's work in porting software, since you have to fix the application bugs which have been there all along but haven't surfaced on x86 but will show up on ARM. Then you have to worry about performance tuning on a different architecture. Plus you have to set up the development, testing, and QA servers to support the new architecture and add it to your release pipeline. And you have to do all this while waiting for years for a significant market to materialise.
Linux distros have supported multiple architectures for years because they have the source code for the applications they distribute and can simply recompile them in most cases. Since portability has been a core feature of most "unix" style operating systems from the early days, most of these applications have had the porting bugs wrung out of them years ago. Debian has official support for AMD64, i386, ARM, MIPS, PPC, and S390x (IBM mainframe). They have unofficial support for others as well, including SPARC.
Microsoft is starting without any of these advantages. Their Itanic port died some years ago and the few third party software developers who bought into that will be shy of repeating that experience until they see a real market running Windows.