Because SteamOS is all about Gabe Newell's issues with Microsoft, and the Windows Store.

Hence they don't have any issue using the real Windows, instead of having an emulation/translation layer for Win32 and DirectX.

For a portable device SteamOS is much better than real Windows though, so the other manufacturers are now at a disadvantage.

I wouldn't be surprised if ASUS is getting kickbacks from Microsoft, since the ROG Ally is quite aggressively priced despite coming with a Windows 11 license, and it comes with a 3-month trial of Xbox Game Pass. And it'd make sense why Microsoft would support device manufacturers - the Steam Deck is really dangerous to Microsoft's Game Pass vision since you can't really use it out of the box there.

broad software library - all those free epic games store games, game pass, etc. And those companies are comfortable releasing a windows device - there's no downside to them at all.

SteamOS isn't yet available for non-Valve handhelds, mostly because that means Valve would need to support a variety of hardware. They've stated they're trying to make it more broadly available now that the OLED has shipped.