The main reason being the cheap resistive display and small, low-life battery. I'm sure the cheap construction and lack of engineering in the case helped control costs. Good luck getting support too. You get what you pay for.
The Viewsonic G looks pretty good, if you blow away the included OS and falsh your own. So for right now it's a geek toy, not ready for the consumer space.
We recently compared the Motorola XOOM with the original iPad and the XOOM emerged victorious.
In that article they compared the Xoom with the expected iPad 2 specs, and the iPad 2 won. They linked another article comparing the yet-to-be-released Xoom with the year-old iPad, and of course the Xoom won. I do notice battery life, an all-important killer feature in tablets, is absent in the comparisons. Any idea what the Xoom gets? In any case, I don't put much stock in highly speculative hardware mashups.
The Xoom does look like a very good product, but notice the price, right in the iPad's range. You don't get a quality tablet for $250.
$249, Barnes & Noble Nook Color, enjoying my preview of Honeycomb right now, and wow, what a game changer for Android tablets. Can't wait until the full ASOP source release is made by Google. Probably will have the sound issues ironed out before then, though.
Oh, and it's a pretty darn good e-reader too.