Most of their success is owing to the invention of new interfaces that fit new form factors. Before that, only dissatisfying shit is available, and the market is mostly novelty. After that, new device categories have actual features a human can use, and a market happens and knock offs compete.
Which interface were you thinking they invented? the WIMP-style interface was borrowed from Xerox PARC. The MacBook was a shiny laptop. The iPod had a menu system much like other MP3 players of the time; they mainly made it a well-organized, consumer-friendly menu system with a bigger screen. The iPhone's main interface innovation I can think of was making the screen bigger; they did use a virtual keyboard, but so did the Palm Pilot and the Apple Newton of years before. The iPad, interface-wise, was basically a big iPod Touch.
Don't get me wrong; those are all very polished products, and they took a lot of technical smarts. They were much more usable to a mainstream consumer audience. But the non-Apple versions of those products were generally fine for non-consumer audiences. And Apple's marketing is masterful; I've never seen a tech company so good at generating hype.
