It was Apple's offering only during OS X, and only because its BSD underpinnings. If Apple had chosen to build OS X on a homegrown kernel, Apple wouldn't have been appealing to the skilled.
I disagree that it was only during OS X. Classic Mac OS, and going back further the Apple IIs, were great for power users.
I also don't understand your counterfactual. Apple didn't build OS X from scratch, they took NeXT's OS and ported it to Mac hardware and tweaked it. The result was something both powerful and easy to use. Making UNIX easy to use is not a trivial accomplishment! Yes, if Apple had done something completely different for their next-generation OS then maybe they would have ended up with something worse. But they didn't.
Not sure why you're being downvoted, the only thing that convinced me to use a Mac (for a while) was the BSD base on some nice hardware. Without it it would be like trying to develop on iOS: slow, locked down and flashy.
