Because Windows is also perfectly fine for running Windows applications & games. It can also be a royal pain in the arse to set up Windows emulation on Linux depending on your graphics card and some other factors.

It's actually easier to get Linux running on Windows since it has WSL. I have Ubuntu running under Windows with IntelliJ open at the moment and postgres running in the background right now.

Inertia.

Because some very important applications (microsoft office, adobe suit, some very popular multiplayer games, cad software etc etc) still don't work.

Specific software requirements for work is the main reason for me.

Also, last time I used linux, it kept breaking, so I had to reinstall the OS about once a month and I had no clue what kept breaking it.

I am in the process of changing before W11 is the only W choice. Many of my steam games work on nix, using the latest Ubuntu now but may need to shop around for another distro, including VR. Except my OGVive won't display, the steam client loads and the mirror comes up and if you move the headset you can even see it move in the environment but... no video on the screens. No VR is a hard deal breaker and I do not have $1000 for a newer headset. Once I get over this hurdle on my laptop I can begin to migrate my desktop.

Others have given a lot of good reasons but those mostly have to do with more business related reasons than casual computer usage. Biggest reason people don’t switch is that the average computer user, who only needs it for casual usage, has no clue how to install an operating system. They simply use what is available at the time of purchase and big box chain stores predominantly sell windows machines. Now and then you may see someone offering a system with a flavor of Linux but that is few and far between. The fact that there are so many variants of Linux is both a benefit to why tech savvy people love it and a hindrance to mass adoption because people like consistent convenience. That is why the iPhone has done so well, each device has the exact same OS and experience. And that consistency with mass adoption means there is a certain level of support that the general user expects. They can go to most PC repair shops and get their windows system fixed no problem but with Linux not every shop is willing to touch the machines so there needs to be more self reliance. So when I say most casual users would be fine with Linux it’s true but for adoption it’s a tricky uphill battle of mass availability of a single user experience that has broad in person technical support.