The real reason why a tracing GC was a failure in Objective-C was due to the interoperability with the underlying C semantics, where anything goes.

The implementation was never stable enough beyond toy examples.

Naturally automating the Cocoa release/retain calls made more sense, given the constraints.

In typical Apple fashion they pivoted into it, gave the algorithm a fancy name, and then in a you're holding it wrong style message, sold their plan B as the best way in the world to manage memory.

When Swift came around, having the need to easily interop with the Objective-C ecosystem naturally meant to keep the same approach, otherwise they would need the same machinery that .NET uses (RCW/CCW) to interop with COM AddRef/Release.

What Apple has is excellent marketing.