In the industry, Dish is known for looking for higher credit customers vs. others like Direct, Comcast etc. They believe that a higher credit customer means they are more profitable over the long term (reduced call backs, missing, broken equipment). Higher credit doesn't mean credit score, could be a bunch of things including how you ended service last time. Did you sit on the equipment etc?
Also, the letter says you didn't qualify for their best offers, not that you could not get service.
Interested to hear how the call goes.