They dominate because Steve Jobs realized what business they're in, and none of their competitors have. This is like when the railroads' lunch was eaten by the tractor trailer because the railroads thought they were rail companies, when in fact they were transportation companies. Samsung and all the other competitors think they're in the device business, but they're not. They're in the UX business.
It helps to have Steve Jobs, who was actually a user experience zealot and in a normal org would have been the CUXO, masquerading as a CEO. As an example, think of the difference between being an Apple vs. Samsung customer, all the way through the value chain. Shopping in that beautiful airy Apple store, experiencing those matte textures and ingenious packaging while unboxing your new devices, the industrial design wizardry that constructed a machine with such amazing fit and finish and tight panel gaps and textures, software that (sometimes, mostly) just works.
Jobs was a strong enough leader to make a bunch of mostly-orthogonal disciplines (e.g. manufacturing, industrial design, software engineering, packaging design) subservient to the company's ultimate objective, which is UX. It doesn't hurt that Apple tends to execute best-in-class on each of these axes, but that's tactics not strategy.
The big question for the future is whether Tim Cook understands this. It's left as an exercise for the reader whether the Apple watch is a product made by a UX company or by a company intent on duplicating the financial success of the ipad and iphone by cargo-culting the "create new device class" methods that worked before.