Marketing, consumer perception, and price. For a competing tablet to succeed vs the iPad it must do one of the following:
1. If it is anywhere near the price of an iPad, it has to be extremely obvious within 10 seconds or so of playing with it/looking at it why it's better than an iPad to a completely uninformed consumer. So far none of the manufacturers seem to understand this.
2. It must be significantly cheaper than the iPad.
That pretty much sums it up...
OTH, since Apple goes the minimalist route with few products they don't cover the spectrum as well as Android. If the rumors of an 8.5 inch iPad are true AND if they have a minimum bezel AND they have a different form factor besides the 4:3 then they can just about nail the lid on Androids coffin in the tablet space outside of the Nook and Kindle.
There will not be a successful mid priced Android tablet because the really cheap Android tablets, sold at a loss (Nook/Kindle), will eat them up. There is room for high end Android tablets but they have to offer something the iPad does not. The most obvious thing is different sizes and form factors and this may be the reason Samsung is shot-gunning with so many different sized tablets -- they're looking to see where they can compete.
I am interested in the Sammy 7.7 and hope the rumors of a 3/1 launch from Verizon are true. This is in my view the best alternative to the iPad owing chiefly to the size and the 8:5 form factor. The ideal pad, for me, would be about 8.5" x 5.5" x 0.4" with an 8.5" screen and 1920 x 1200 resolution with a BIG battery.
Brian