Because organizations with specific needs and significant resources often benefit from writing their own framework to exactly fit their use case. Consider Facebook, to a first approximation, there is only one framework. Google is in a similar position, though there may be more than one framework, there are not many frameworks.<p>For organizations without significant resources, JavaScript frameworks can be a bit like wearing someone else's underwear. Technically it works, but you're probably looking for a better alternative.
Nothing wrong with having many choices but to answer the question I think it's because of the sheer of popularity it has. I mean it is the language of the web after all.