The company claims that it did not support MacOS because there were not enough gamers out there to justify it – Apple made up less than one per cent of active CS:GO players.
However, a more obvious reason is that Apple refuses to support the Vulkan API that the game is based on.
Vulkan was designed to succeed OpenGL and address some of the latter's shortcomings. While an open-source library called MoltenVK provides a Vulkan implementation on top of Apple's Metal graphics API, it still lacks some of Vulkan's advanced features.
Counter-Strike is not the only game which will go this way. Developers will not design desktop class games around API for a single platform. Recently, Blizzard dropped macOS support in their newest titles.
Meanwhile, Apple seems to believe that game publishers will automatically make a Metal-compatible version just because it says so.