AppKit is now very old, and maintaining backward compatibility through annual release cycles is extremely complicated and challenging. And that complexity doubles when you add a new platform in the mix.
Porting AppKit to ARM is easy. It's another thing entirely to commit to supporting parallel versions of AppKit for the next 8+ years.
It would be a colossal waste of Apple's software engineering resources, which many people believe are already stretched too thin. And for what, a little better battery life? It's just not realistic.
