The Nook Color has sold 3 million in the US as of March 2011(it achieved this in around 6 months) and is only available there. IMHO,if Amazon sold a decent tablet it would probably be quite successful IMHO.
Edit!!
It seems a Nook Color 2 is being released:
http://thenextweb.com/gadgets/2011/0...ets-to-market/
Amazon would want their tablet to be competitive with the Nook Color 2.