It’s because Apple keeps shooting itself in the foot gaming wise. Instead of embracing open standards they made Metal. Instead of keeping Open GL up to date they let it languish for ages until finally Metal came out. Then the transition to Apple Silicon and dropping all 32 bit support killed a ton of old apps.
About 10 years ago Mac gaming was far better than it is today. Pretty much every game that ran on Linux ran on Mac.
Linux took Macs spot less because it’s gotten so much better, and more because Apple has shot itself in the foot. Without the steam deck Linux would have overtaken them eventually but mostly because they’re bleeding gaming market share.