There probably is no one single factor that has made the iPad the success it has been.
Price, performance, design, supply chain, quality, retail, ecosystem - if any of them don't work, the whole value chain would fall apart.
Example: You could build an affordable, easy to use, brilliantly performing piece of hardware - but if there is no software to run on it, there is no real reason for consumers to buy it. Or even if there WAS software, but quality was a problem, consumers aren't likely to take a chance on a new device.
In my opinion, Apple is the ONLY company in the world that could have made the iPad work. Can you see consumers lining up to buy a piece of hardware from Microsoft? Can you see App developers rushing to create paid Apps for a Microsoft Tablet App Store? How about Sony, or Samsung?
Apple was in a unique position: They'd done it AT LEAST twice before in the previous decade. They'd convinced not just consumers, but also developers that their company could introduce a new piece of hardware and, in very short order, build an entire new business. More to the point, most of the infrastructure to make a new product succesful (the App Store, the customer accounts, the retail stores, etc.) was already in place.
Other companies might be able to copy different parts of Apple's "value chain" - but no company, by itself, can replicate much more than one or two of those links. Google could put out the greatest iteration of Android for Tablets ever - but if the engineers at Motorola or Samsung don't do their homework, the whole enterprise fails.
One immediately glaring shortfall the "me too" companies are suffering from is Apple's commanding lead in the supply chain. By utilizing Asian subcontractors for assembly and component supply, competitors can - at best -match Apple's manufacturing cost. But Apple is already two steps ahead: they've used their inside knowledge and financial might to lock up supplies of the key touchscreen manufacturing infrastructure. Even if a competitor was able to produce a credible alternative - they wouldn't be able to get enough touchscreens to meet the demands of a "breakout" hit. Apple doesn't "monopolize" touchscreens - it just buys enough of them to make it very expensive and difficult for competitors to source them.