Especially when used in a browser environment, there are no threads (let’s ignore the rather new WebWorkers for now) available – all JS runs in a single thread which is usually the same one that also handles e.g. rendering of the page.

So performing actions in a blocking manner is simply not an acceptable option – especially since most actions that involve IO in JavaScript usually cause network IO which, unlike disk IO, is usually rather slow and thus blocking everything until a request finished is not acceptable.