I don't remember how officially it was stated, but original push by Valve for steam on Linux with Proton was to remove their dependency on Microsoft - a hedge against possible future ecosystem-impacting decisions in Redmond.

Making SteamDeck use windows wouldn't impact prices much, Microsoft is really friendly for putting windows by OEMs. Could even run modified to act like current steam deck.

Instead, SteamDeck is there to drive up testing on Proton or straight forward porting to Linux, which just availability on Linux and the previous steam machine didn't drive up