Margins.
A netbook that gets better every year for $250-350 will have a razor-thin profit margin. If it has to have Microsoft Windows Special Edition on it, 10-30% of the price tag goes to MS. Joe Average wouldn't buy a computer that doesn't run whatever brand name he recognizes, so Windows is a requirement.
A tablet isn't a "computer", so it doesn't need to run Windows in order to sell, it needs to run Facebook and Angry Birds. It can sell for $250-350 and have a decent margin, and then there are so many of them being made that the parts cost drops and the margin gets better.
A Google Nexus 7 now has everything except a keyboard in terms of netbook hardware, but it gives up no margin to MS and has a market big enough for economies of scale.
Meanwhile, whatever Intel is pushing as an Ultrabook can go for $1000-1400 because it starts as a high-end desktop replacement and then gets features shaved away. Except nobody with any sense buys them, and the usual bulk purchasers of laptops that don't make sense (Fortune 500 companies) aren't re-buying machines as often, and when they do, sometimes it's a fruit machine in offices which would never have bought those five years ago.
(Why? because of you, dear HN reader. The alpha suits know that the alpha techs demand the best hardware, and they can see the logos glowing on the back of your screen better than you can.)