I think those numbers also reflect something other than the obvious.
There's a huge contingent of…uh…let's just say "differently educated" developers who previously were making some delicious pasta in PHP and now want to build iPhone apps because they heard there's gold in them thar hills.
Objective-C is also a lower-level language than CoffeeScript and thus harder to use, especially if you're not used to working in a language where it's possible to free memory early or smash the stack.
So CoffeeScript is a very easy language that tends to be used by developers who are experienced in its domain, while Objective-C is a relatively hard language that has gotten a huge influx of rank amateurs in Stack Overflow's lifetime. Thus, the latter gets a disproportionate number of questions.
(For credibility's sake: I'm one of the top contributors to the Objective-C tag on Stack Overflow.)
Other factors probably at play: Obj-C MUCH older. CoffeeScript designed to be simple and have fewer "bad parts" thus fewer questions. Many problems faced while coding CoffeeScript are actually javascript or DOM or Web questions so you don't tag them with CoffeeScript. StackOverflow isn't overly loving of CoffeeScript, so I often ask my questions using JavaScript even if my source is really CoffeeScript (and again the questions aren't really about CoffeeScript but usually about JavaScript).
Objective C has been in heavy use for a lot longer then CoffeeScript though. CoffeeScript has only been stable for over a year. Objective C has been in use since before the creation of StackOverflow.
You have to factor in all the newbies that here all this doom and gloom about the job market in the media, and the last tidbit in the news story usually mentions "but there is a lot of growth in iPhone and Android apps". This is probably causing a lot of people new to programming to learn Objective-C, thus many more questions.
That could also say something about the languages and the platforms rather than the number of people using them, especially since all of Apple's libraries fall under the category of Obj-C. You're probably right, just wanted to point that's not necessarily a good measure either.
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