There are many cheap 5" and 7" tablets. But very few 10" tablets. ViewSonic gTablet or viewpad10, Archos 101. They all have pretty good hardware with many of them with two cameras and comparable price with low end iPad. Processor manufacturers like nVidia gives them good frameworks to start with. In addition to several things mentioned in the comments, these tablets had problems like,

1. Almost all except Samsung Galaxy lacked direct use of Android market. 2. Some had software problems with bad out of box experience. (For example: http://www.engadget.com/2010/12/18/viewsonic-g-tablet-pulled...) 3. Regular apps built for Android phones did not give good experience on the tablets

For the future tablets, waiting for Android 3.0 may not be a good reason. They always can release the hardware and then update the software later. HTC flyer is following this approach. There is a risk of users turning away if the first impressions are really bad. Announced prices for several of them are on the very expensive side. Among the announcements I also notice that very few come with wifi only option which is another reason for high price.

I collect tablet information at tbltpc.com.