If I had to guess, the reason WebGPU is not implemented in Linux is because nobody is paid to add features to Linux. This means that new features are delayed by several years. WebGL is old, and well supported. WebGPU is newer, and has less support.
Google cares about Chrome Linux support and pays people to work on it, for two reasons. One is Chrome OS, and the other is that most Google engineers use Linux desktops for work.
Do they? I thought they would mostly be on MacBooks.
Most Google engineers have both. Or at least they did when I was there. And Chrome developers in particular do their development primarily on Linux.