[petabyt]

Some of the compilers they use (msvc) are not open source and are not allowed to run in wasm. And porting entire compiler toolchains to wasm would be a gargantuan task.

[charcircuit]

WASM has x86 emulators.

[tempay]

It would be a lot of data to download and would likely be uncomfortably slow for many cases.

Also, as is mentioned in the article in a few places, the website is optimised for low effort rather than low cost.