Apple merged with NeXT in the ’90s and Mac OS X was made from NeXT’s operating system, NeXTSTEP. Objective-C was the official language of NeXTSTEP’s application frameworks, which became Mac OS X’s Cocoa. Mac OS X was then adapted into the iPhone OS, and Cocoa was made into Cocoa Touch. Objective-C has held up pretty well all along the way, and a lot of Cocoa’s features would be difficult to translate into C++.
So essentially, it all comes from NeXT.