Those platforms started that way in the first place, it’s not like iOS/Android/Xbox/PS4 were open and then changed it later. SteamOS is probably not going to be successful, and the reason for support is most likely simply having Linux support over them trying to support SteamOS.
The issue is that you shouldn’t be forced to use the Windows store just to get the advantages of UWP. I’d rather not have to give up 30% of profit along with having to submit to approval any time I release something or update. Microsoft would already benefit from it if someone develops a product that could then go to all of their platforms.