There was no lack of games, Valve even needs to emulate DirectX and Win32, as Android developers couldn't care less about Steam Deck.
The rewrites across multiple versions, that is what killed the desire to target the devices.
The rewrites across multiple versions, that is what killed the desire to target the devices.
I don't understand what you write about Valve, the Steam Deck supports unmodified Win32 games which use DirectX.
The effort to port between Windows and Windows Phone was way less than between Windows and XBox.
Besides that, porting wasn't easy even for apps which worked on Windows already. The API was quite different, and not in a good way. For example none of the standard Windows file I/O API was available. There was DirectX but it was DX11 FL9_3 which is not how you access GPUs on desktop Windows (you would either use DX9 or DX11 FL11_x).
It was still DirectX and Win32 for the most part.
Only FOSS indie devs make such an uproar of OpenGL and friends.
Walk around GDC, PAX, GDCE to see how much professional game developers care.