Atwood's Law [1] ... Mainly because the web browser is the single most broadly available interface with JS being the most accessible language to use in that interface. There are definitely other JS x86 emulators out there. JS isn't the fastest available, but it's far closer to run everywhere.

[1] https://blog.codinghorror.com/the-principle-of-least-power/

Mainly for fun and because I like this language. Also, my goal is to let people drop a x86 demo in their browser and just watch it play or interact with it without any other tool.