Primarily because it's an extremely profitable product that's selling as fast as Apple can manufacture it. HP, Google, and Sony already have their copiers cranked up.
MS may end up being the odd man out here. They've had little luck in scaling their products into a small light footprint. HP killed the Slate, but will probably bring it back with the Palm OS. This could be interesting. If Palm had leveraged their Pilot into a phone they could have had the iPhone quite a few years before the iPhone. HP has the ability to make it work.
I do not believe these tablets will be a fad. The head of Nintendo recently told his company engineers that the competition is not Sony. He believes they've won the battle with the Playstation. He believes the next battle for the gaming platform will be with Apple.
My neighbor bought one last week. I must say, there's something unusually comfortable about laying back in the couch and surfing the net. I do it know with my iPhone from time to time, but that really is too clumsy to do any power-surfing; it's just too small. In any event, while the lack of support of flash is a deal-killer for me, I could see how these things could really take off.
If something that Apple will support replaces flash as the preeminent video delivery platform in the near future (which seem highly unlikely), iPad could really establish for themselves the dominant position in a rapidly growing market. It will be interesting to see who blinks first, Adobe or Apple.