Earlier quoted context omitted.
Because video-games is still the one place where Mac and Linux are lagging behind, and they have DirectX to thank a lot for that.
In case of Linux, you forgot availability of drivers, cohesive user experience, stability and ergonomy rather than programmer designed interfaces.
First some context : I'm both a Windows, Mac and Linux user, and my favorite is probably Mac. But I've converted a fair share of my family to Linux (my grandma too) because it's more predictable than Windows, and cheaper than MacOS (I'm basically my family's IT hotline). Most of what I'll talk here is a mix of my own experience, and feedback from my family members.
Let's talk about drivers. Turns out, drivers are available for most of the hardware you'll need (which is basically GPU, WiFi, Bluetooth and Webcam). Sure, this weird piece of equipment might not have linux drivers, but in truth I can't even remember the last time I've hit that problem. I don't think equipment compatibility is the reason people use Windows in 2017 anyway, given we're all using Android/Linux on our smartphones and tablets, so I'll move on.
Let's talk about cohesive UX. Windows, since 8 and 10, has the least cohesive UX ever. Running Gnome or KDE on Linux today will get you a more cohesive UX than win10, and that's no compliment. Half the Windows apps still use the old winforms look, half use the new metro (or whatever it is called) look, and then there's all the webapps that have their own custom look and feel. Cortana is a clusterfuck of an UX, the File Manager has this weird "It's a filesystem but not really" mentality (the location bar doesn't show the full hierarchy, WHY ?), and there are so many papercuts here and there that I end up fighting my OS half the time.
Let's talk about stability. Windows forces reboots every few days because it has "mandatory updates". Windows basically built this feature where they force the computer to reboot and the user has absolutely no say in the matter. This is not the stability guarantee I'm looking for. Another anecdata : one of the few family members to still have a windows has a Microsoft Surface Pro. She has to reboot it every few days because it won't detect the Wifi card anymore. Microsoft can't even make their own OS stable on their own hardware.
Windows might have been good at all of this at one point, but that's not the case anymore. It's become terrible at all of it. So bad that Linux slowly creeped its way into my family, and that made my life much easier.