For sure, it's amazing to get a full kernel running; there's so much one could do. But an emulator and a full kernel is going to have a sizable footprint, be a significant amount to download, and performance is going to be mediocre. Yes there's a linux kernel running, but it's inside a virtualization layer; if you want to integrate & have the page interact with the kernel, you'd need to start hacking some pretty wild channels to punch through that virtualization layer.

Writing an operating system that directly targets the browser, that offers the expected/standard system calls ought to be smaller and faster, and more interestingly, it can potentially integrate with the rest of the page in interesting fashions, easily.