Comment by ndsipa_pomu

It was likely about control. Valve saw that Microsoft was becoming more controlling about the Windows platform and that's what pushed them towards developing SteamOS on Linux as that means that Valve can put resources into fixing anything that they want to. The Apple platform is also under control of a single entity, so it doesn't make too much sense for Valve to care about that (as well as Apple not being known as a gaming platform).

What you should do is just buy a SteamDeck for gaming.