Gaming mostly, and inertia. But I feel like the choice of OS is a much smaller factor than it used to be. I have a Windows desktop and a MacOS laptop now, but I've used a Linux (Debian Gnome) laptop for many years and I used to have dual boot on my desktop.
Everything runs on everything now mostly, and the rest are web apps.
Chrome, VSCode, Obsidian, Spotify, Steam, Slack, Discord, Docker, Python, Rust, JavaScript (Node), decent terminal (Bash, Fish, Powershell)... That's pretty much all I need, there's almost no difference between OSs on all these nowadays, most data is synced, and I can switch between them seamlessly throughout the day.
The UX is also very similar between all modern OSs, actually Windows 10 is now quite clean and snappy, I might slightly prefer it to MacOS and Gnome.