Compile-to-JS is a leaky abstraction that gets messy when JavaScript's type system and built-ins collide with your language's. Some languages also need specialized tooling for their compile-to-JS stuff, rather than treating it as just another compile target.

I usually find it causes more problems than it solves, but I'm hopeful that someday WebAssembly can make compile-for-Web transparent enough that non-JS languages really feel like a great experience in the browser.