As far as I can tell (as an amateur mac developer), the major advantage Objective-C has on the mac over other platforms is the extensive support that Cocoa [1], Apple's giant grouping of APIs/Frameworks/etc, lends. It's so helpful, in fact, that many Mac developers refer to programming in Objective-C as "Cocoa Programming" (also, it's the title of the best programming book I've ever read [2]).
Sounds like it's branded/positioned as "Cocoa"... and possibly support is lacking on other platforms (I guess you mean libraries - they're crucial for language success).
[1]http://developer.apple.com/cocoa/
[2]http://www.amazon.com/gp/product/0321503619