For most consumers a internet set-top box for TV is a completely new type of technology purchase (similar to a tablet purchase). Set-top boxes don't have the "cool-factor" of other technologies (smartphones/tablets) to drive sales, and TV === Cable mentality is already ingrained, so the barrier to adoption is price. The price needs to be low enough that the consumer is willing to take the risk of adopting a new (possibly dust-collecting) gadget.

E.g. I really wanted a GoogleTV, but the price-tag was high for something I was sure I'd use a lot. Instead I got an Apple TV for $99. It doesn't have everything I want, but it suits my needs 90% of the time. (also see eBook readers)