There are a lot of web miners - based on WebAssembly. Most of them mine Monero. Is there any special reason for that?
Monero is one of the coins which use an ASIC resistant PoW algorithm (Cryptonight), which means ASICs for it are expensive to make, so CPU have a good shot at mining it, contrary to most other coins, where ASICs and/or GPUs leave CPUs with pretty much no chance.
Two reasons I can think of: addresses aren't traceable and its still (though barely) profitable/mine-able.