If it's done right: easier to do simple things with, faster development cycle, run time evaluation of code... all the usual reasons why people use interpreted languages instead of compiled ones.
Objective-C is a very dynamic language, especially among compiled languages. It was based on Smalltalk, the same language Python and Ruby were influenced by.
There is really very little room for JavaScript to improve upon Objective-C.
(It's also worth noting that Objective-C itself may well be as popular, or more so, than most of those same languages. It is, of course, a difficult thing to measure, but there is some evidence to suggest its at least more popular than languages like Lisp)
Still, though, you can do some interesting things with interpreted languages, like eval()...