Why? It's superb. It's available to nearly everyone with a web browser, easy to pick up and learn (deceptively so), and allows you to do things with webpages that would be impossible otherwise.

Yes, it has flaws, and lots of people use it without really understanding how it works / best practices etc, but purely for it's accessibility and ubiquity, coupled with its charming quirks and idiosyncracies, I think it's great.

It baffles my mind that more people don't appreciate Javascript for the lovely little scripting language that it is.