The vast majority of people buy the iPad not because they want a tablet, they buy it because they want a really big iPhone. HP, RIM and most of the others seemed to not understand this and attempted to build a really big iPhone... without the iPhone ecosystem. Surprise, surprise they failed miserably.
Apple did a very smart thing, they took a product that was selling like hotcakes, and they introduced a product that was identical save for the form factor. Every iPhone user (which is a whole lot of people) was suddenly an expert iPad user! They felt at home, they felt safe, they got what they wanted: a really big iPhone.