I'm a bit out of my depths here, but I believe a significant contributing factor is how early Apple made multi-CPU Macs available, with the earliest being the s

I'm a bit out of my depths here, but I believe a significant contributing factor is how early Apple made multi-CPU Macs available, with the earliest being the summer 2000 revision of PowerMac G4 tower (dual 500Mhz PPC G4s), pre-dating the release of OS X. They made it easier for devs to take advantage of those cores in OS X, because this yielded performance boosts that were difficult to match in the x86 world, which was still heavily single-CPU. Because the OS and apps running on it were already taking advantage of multithreading, making them efficiency core friendly was easy since devs only had to mark already-encapsulated tasks as eligible for running on efficiency cores, so adoption was quick and deep. Meanwhile on Windows there are still piles of programs that have yet to enter the Core 2 Duo era, let alone advance any further.