Valve wanted steam to co-exist on the mac in the early days and John Sculley of Apple didn't want Apple to be seen as a gaming device or a "personal home computer". So they ceased contact with Valve and the rest is history. https://www.youtube.com/watch?v=lPTLPXNtb2I
Apple refused to license joysticks so they could prevent customers from considering early mac's as game machines and deliberately refused to support games on the machine. Myst was only few that were exclusive to the Mac; that they then ported to PC.