I think there was an article on this (and maybe a thread on it). I think the article said that Directv and Comcast didn't want to support the Roku app because they considered Roku as a competitior. I agree that Dish made the better choice by supporting it.
Dish might have realized they have to embrace on-line distribution or maybe lose customers all together. An over the air antenna with digital TV and a Roku is starting to make a lot of sense compared to $100/month+ cable/satellite bill where people are forced to buy a lot of channels (and sports tax) they never watch. Dish has Blockbuster now but does not have a box beyond its own receivers so possibly they are looking to make distribution available via Roku.