Even if your assumption Objective-C was only relevant after the release of iOS was correct, then Objective-C was still relevant since 2007. That's 7 years.
Check this out. Objective-C wasn't even Apple's first choice.
"Apple CEO Gil Amelio started negotiations to buy Be Inc., but negotiations stalled when Be CEO Jean-Louis Gassée wanted $200 million; Apple was unwilling to offer any more than $125 million. Apple's board of directors decided NeXTSTEP was a better choice and purchased NeXT in 1996 for $429 million, bringing back Apple co-founder Steve Jobs." https://en.wikipedia.org/wiki/BeOS
So you're all using Objective-C because Apple couldn't afford to buy the OS they wanted. Kinda sad!
In the early Mac OS X days, developers could choose between Objective-C and Java as the Mac OS X main language, the majority went for Objective-C and the JavaBridge was dropped.