Source is this video:
Windows Was The Problem All Along - Dave2D
We could obviously compare performance between windows and steamOS before on the steam deck, or between windows and Bazzite on other handhelds. But this is the first time we have had official windows and SteamOS builds for the same hardware.
That my friend, is entering operating system politics.
But the TLDR is: resistance to change, lack of support, bribery, a combination of all 3, features, and much much more!