Comment by WorldMaker

It's the same problem that you have, isn't it? You don't want to support two codebases, you just want to build your app once and have it run in two places. It costs time and development budget to maintain two different codebases, and you have to prioritize. Can you really blame Microsoft that they don't want to maintain both a modern codebase and a completely separate fork/back-port of it for a much older branch?

That said, it has been possible to share a lot of code (if not almost all of it) between a WPF and UWP app for a while now with PCLs, or after that targeting .NET Standard. That gets even easier once UWP support for .NET Standard 2.0 ships soonish. There's also been work recently on Xamarin.WPF for Xamarin's cross-platform code sharing, and the XAML Standard 1.0 work trying to converge much of the XAML across all the platforms Xamarin supports and UWP to get rid of a some of the dialectal nuances.