Note: It's worth noting that Kindles absolutely dominate Amazon's best seller, most gifted and most wished for top twenty lists under electronics, but are nowhere to be seen in in the two twenty top rated electronic items.
So there's already massive demand. What's the second reason? Price.
That $199 price tag is sweet. That's the same price as a 4GB Xbox 360 on Amazon, $50 cheaper than the 160GB PS3, cheaper than any other tablet (unless they're junk), and a whole lot cheaper than any PC or Mac. Under $250 would have been sweet, but being under $200 adds a cherry on top.
Finally ... and this is important ... no one has a Kindle Fire yet. Unlike the iPod (where everyone who wants one already has three), all and the games consoles and computers and tablets, the Kindle Fire is new and exciting. That freshness will be a huge selling point (just like it will be for the iPhone 4S)
One caveat to my 'Kindle Fire will be a huge hit' thing - that Amazon has the supply chain figured out and can keep up with demand. If the Kindle Fire goes out of stock within seconds of release not to be seen again until after the holidays, then I guess Amazon's blown it.