A Mono-a-like is not required as Apple has fully open sourced it. Instead there's GCC, Clang and http://www.gnustep.org/

Well the entire stack is open source so no one needs to port it. I'm pretty sure that people like it because, it along with cocoa, makes iOS a great platform to develop apps for. But as for spreading objc, most of the features it has have been in dynamic languages like perl for many years longer. Perl even has objc lib bindings and much better integration in unix, albeit not necessarily for making apps.

I think the popularity of objc is hindered more by the fact that people would be moving a step back, if they were using it for something besides what it's designed for (cocoa apps).

My point is that the existence of ports is irrelevant. A language is popular if it is used by a lot of people. The choice to use it involves a lot more than language preference.

To claim that objective-c doesn't count as popular because anyone who chooses to write for iOS must use it whether they like it or not, is like saying that java doesn't count as popular because anyone who chooses to develop on an emterprise stack that a corporation has built on java must use it.