There are a number of reasons why Apple is running away with the iPad while it's competitors choke in it's dust trail, but the one that gets missed out all too often for me is the user population. It always seems to me that most folks are too eager to roll phones and tablets into the same gestalt-übermarket, but this to me seems completely unfair. Consider that in the UK there are 60 million people (give or take a million) and 80 million mobile cellular subscriptions. Now while these subscriptions include mobile data contracts (including 3G iPads) you still have to face the fact that there are more mobile phones in use in the UK than there are men, women and children of all ages*. There are fewer tablets than this globally! (though that should change in the next 12 months)
The mobile phone industry is relatively unique among tech markets in that it has always been highly competitive and it has always been colossal. Nokia, RIM, Samsung, Sony Eriksson and the others have historically competed quite well for a slice of the cyclopean cellular pie. It developed over more than a decade and mobile phones managed to become one of those things that nobody can operate without. Even when the newcomer Apple entered the fray there was plenty of space for everyone. Brand loyalty and an expectation of choice are long-term staples of the phone-shopping experience. If you look at the division of Android phones (discounting Apple and the self-exiled Nokia) this competitiveness has continued. The mobile phone market never really had to change to accommodate the change in technology it's merely a matter of dominance shifting.
The tablet market however isn't a market for big-screen phones. It will never be as big as tablets will never be as must-have (though it is displacing the home computing market). It is in fact more comparable to netbooks and small laptops/notebooks in which Microsoft has ruled the roost for a long time and while the hardware manufacturers enjoy a situation similar to phone manufacturers it is software that undisputedly commands the personal computing experience. So to simply assume that this fledgling tablet market would behave like the smartphone market because the operating systems are the same is shortsighted. Getting in first and getting it spot on right out of the box, defining the market itself and capturing the expectations of the consumer meant everything. Now you can't dislodge Apple from the top of the hill by playing the game with the rulebook they wrote. This was demonstrated previously as the initial round of Linux (chosen for performance) netbooks were rapidly overtaken by machines running Windows XP which offers a much better and more familiar use experience for most people.
It's little wonder to me then, that HP and others are seeing less than awesome sales of their 'like an iPads'. Why settle for anything less than the leader of the pack in such a small market? It's not a case of the iPad being the most awesome thing ever, and it's not a case of cultish mind control; the contenders need to offer something other than that which Apple is already providing in a category of device that is so new. The market, as with portable computers, is polarised and the way to break into it is to do what Apple did a little over a year ago and change the game; Apple never threatened Microsoft's superiority and so they took away (or are slowly undermining and eroding) the market Microsoft built.
* Numbers taken from Wolfram Alpha queries.