PC Gaming

The goal was to not lock down their hardware to someone else's operating system, it wasn't just to sell games on steam. Valve has always been very open about not wanting to be beholden to other corporations for their success and innovation. They realized they could have their own software and hardware platforms that work in tandem more efficiently than slapping windows on the deck due to owning both pieces of that puzzle. Not to mention they were paying modders under the table to help them build dxvk and integrate it into proton.

No other handhelds have adopted it most likely because the drivers aren't updated for the latest chipsets, which means all efficiency goes out the window (no pun intended). Sure you could probably run the updated Ryzen graphics on SteamOS but it probably wouldn't benefit the hardware as much as Windows would, considering most power efficiency on Windows is handled more by the OS and not the chipset drivers.

At this point what valve needs to do for more OEM adoption is provide better compatibility for SteamOS's integrated power management tools and expand the compatibility for SteamOS outside the steam deck hardware. Until that happens these handhelds will just opt for Windows instead because it's less overhead on the OEM partners.