Understanding Objective-C's dynamic runtime

I'm getting my feet wet in Objective-C and Cocoa and I noticed that all objects are allocated out of the heap.

Is there any reason why this is the standard in Objective-C? I'm especially keen because Objective-C is considered a strict super-set of C with OO features.

I think this owes to the Smalltalk heritage.

Also, you could say that stack-allocated objects are a peculiarity of C++. Very few other languages do that, and then mostly as a low-level optimization for the compiler. In C++ it's reasonable, since one of the stated goals was to be exactly as performant as plain C.

Why do you feel this is in conflict with the fact that Objective-C is a super-set of C? After all, plain C can't either allocate objects on the stack... nor anywhere. structs, OTOH, are allowed on the stack, both on C and Objective-C