While, from my point of view, UWP is what .NET 1.0 should have been (see [0]), the way it was introduced, requiring a complete rewrite from Win32/.NET applications, with limited capabilities, already required some adoption love.
Then while trying to achieve feature parity with Win32/.NET, this was done at the expense of breaking compatibilty across WinRT generations (8 => 8.1. => 10 => WinAppSDK), and if that wasn't enough, also breaking the .NET and C++ tooling experience in the process.
I think it was a mix of issues, management naturally, lack of empathy for the Windows developer community, whatever internal KPIs they had/have to meet, and most likely also quite relevant, it appears new employees don't have much experience with Windows development or its culture, were raised in a world without Windows.
People like myself have been PC users since the early days of MS-DOS 3.3, naturally new employees now have other backgrounds.
[0] - https://arstechnica.com/features/2012/10/windows-8-and-winrt...