LLVM/Clang has been a huge boon to the GNU/Linux ecosystem, and dragged gcc into the modern era, and was largely funded Apple for many years.
It's better to point out WebKit.
It would seem that you never used GCC in the pre-llvm era. The GCC project has had a couple of notable periods of stagnation, in each case being "rescued" by the emergence of meaningful competition. First EGCS, and then later llvm.
Clang brought new developers to the space, it disproved the assertion that error messages had to be cryptic and unhelpful, and it has been a peer competitor for an extended period of time now. The two projects compete and cross-pollinate to their mutual benefit.
Not to say they've fallen behind, just that a competitor clearly kicked things back into gear. Same as Firefox and Webkit-based browsers did for the many years of IE6's monopoly.
If you use Windows or macOS then you can open all your time using the OS suppliers tools or tools bought from third parties.
In macOS you can use UNIX but it is the FreeBSD world so no GNU or Linux there.
In windows if you want Unix then WSL which is GNU/Linux but you can easily work in plain Windows.
If you're building software you're often going to target your audiences where they are.
In the developer world, there are many desktop Linux users. So, we have more tools available to us related to our jobs. I use desktop Linux and appreciate that.
But, most people across industries and in their personal lives are not in the same situation.
GitHub, TypeScript, Vscode (when I have to interact with people who cannot think outside an IDE), and I've just started to use Playwright.
Sadly there isn't much I can do about TypeScript and NPM.
Microsoft wants to control everything in our lives.
The reasons, in decreasing significance: (1) demand and (2) OS interfaces.
1. Linux desktop has a small user base; there's simply less interest and value in porting to Linux.
2. The POSIX interface is basic (well, more basic than Windows); it's easier to emulate/replace/shim Linux interfaces than Windows ones.