That's because there isn't a vast SDK like Cocoa outside of OS X. That needs vendor support.
But that doesn't say the whole story.
Objective-C started as a small language, with a commercial compiler by Brad Cox and no backing by any major vendor. Since then it has only gone from strength to strength.
Before Apple, it was used by NeXT. The first major first-person-shooter (Doom) was done by Carmack on a NeXT in C and Objective-C. The first web broswer was written in Objective-C, by Tim Berner's Lee. Both have praised the environment and language.
Then, there was the OpenStep initiative, between NeXT and SUN, again using an Objective-C API, which, have it been seen to fruition, it would have given us a great UNIX desktop/workstation environment, like a cross between NeXT and Solaris, a decade before OS X and modern Gnome/KDE.
Then there's also the GNUStep project, a third party implementation of that OpenStep/NeXT idea, that also maintains an Objective-C compiler.
There are even ...Javascript ports of Objective-C ( http://www.cappuccino-project.org/ ).
And now of course, Objective-C is adopted by Apple after acquiring NeXT. They sure didn't feel it's a bad language and they have to move on -- instead they bet the whole company's development plans (and third party devs) on the language, and it paid off.
It's one of the most mature and development environments. And I've never seen anyone that programs in it (as opposed to "tried it for a week") badmouth it -- quite the contrary.