This was no accident. Look no further than the Apple TV: Often dismissed as a little more than an experiment, the company's set-top box has in its lifetime been powered by specialized versions of both OS X and iOS. (The latter is used in newer models, since, given the device's specialized nature, it doesn't require more involved, more expensive hardware.)
While the underlying operating systems are the same as those running on a Mac or an iPhone, however, Apple built a completely different user interface designed to work well with a TV remote instead of expecting users to equip themselves with a keyboard and mouse—or, perhaps, to get off their couch and start dragging their fingers across their 50-inch HDTV.
Specialization is good
The underlying idea behind all these choices is that specialization and focus help make better products—which, in turn, brings me to another reason that a unified operating system would be hard to pull off: the ecosystem.
Meet any developer who works on Apple's platforms, and you are likely to find someone who sweats every last detail of the way users interact with their apps. Needless to say, this requires a massive time investment into perfecting both functionality and user interface—an investment that often must be repeated if you want to support phones and tablets, or even phones of different sizes, even when they all nominally run the same operating system. Building a Mac version of an iOS app is only occasionally on an iOS developer's radar, and then only because it can be built—and sold—as a separate product on its own timetable, rather than as part of the iOS app itself.
But to expect Apple to suddenly throw a desktop-type interface into the same mix, and then require developers and designers to support that alongside touch-based interaction in a single package is a little ridiculous. That suggestion speaks to a particularly noxious attitude that some non-technical people seem to hold for software development, regarding it as little more than an afterthought. I can't speak for everyone else, but I don't conjure working code out of thin air by waving a wand over my keyboard. And while Apple may have the resources to attempt to build an "iAnywhere" operating system, I doubt that a large portion of the software developers who populate the company's App Store would be able to do the same with their software—particularly at the cutthroat price points that users now expect as the norm.
More equal, and more different
Apple's executives have been unusually chatty about the differences between OS X and iOS in recent months. In the company's Jobsian past, this could maybe have been taken as a signal that Cupertino was busy working on exactly those products that its CEO openly denied it was interested in.
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