Other languages have made it into the browser, in much the same way that other languages than raw machine code have made it into computers in general.
JS is the common compilation target of the web, and also happens to be a language that many programmers find tolerable enough to use directly.
As nice as it is to have multiple options of source languages for the web, there's a lot less value in having multiple options for compilation targets, as well as greater costs (particularly given how tied execution engines are to browser engines, so that you really need a separate engine for each compilation target for each browser engine. Having a reliable, good-enough cross-browser target language that is a the intersection of the JS supported by each browser engine is easier than doing that same thing for multiple different target languages.
A new target language is going to have to offer really compelling improvements in execution to make sense to compete with JS in that role on the web.
