Android tablets have nowhere near the market share that Adroid phones have captured. Why is this? Well, John Gruber, in Daring Fireball, speculates as to the reason:
My hypothesis has long been that Android has very little traction in and of itself. What has traction is the traditional pattern where customers go to their existing carrier’s retail store to buy a new phone, listen to the recommendations of the sales staff, and buy one of the recommended phones. Tens — hundreds? — of millions of people have done this and walked out of the store with a new Android handset. (By my theory, this is why Android phones are so under-represented compared to the iPhone in terms of usage — things like mobile web traffic. A lot of people think of them just as phones.)
There is no such traction for the idea of going into your phone carrier store and buying a computer. That’s why carrier-subsidized netbooks didn’t take off, and that’s why carrier-subsidized Android tablets haven’t either.
I think the other reason may be marketing. Most people couldn’t care less about a device’s specifications, much less even understand them. However, Android Tablets are being marketed as hardware. Contrast this to Apple’s marketing of the iPad. They market it as an ecosystem. Big difference.