Because they know it's still a great device and they waited a year just so they can get it for cheap from desperate sellers. They don't care so much that a new iPad is coming out - it's more that they'll be able to get so much out of this great device for a bargain (in their eyes). And it's true.
I would venture to say a jailbroken iPad 2 will be better than a iPad _ out of the gate.
And there's also the dual-core processor, the front-and-rear facing cameras for skype/video production, its use as a video/movie player, the amazing screen that makes playing apps and browsing the web so much more easier (I miss my paddy
...).
They're the smart ones. We are risking it by selling our older iPad in hopes that the new one will blow our socks off.