Well the coined response is that there is a lack of compelling content there. But there is also a lack of bandwidth, and the other cost issues etc...
Dish has been testing putting 3 HD channels per transponder for a while but nothing has seem to come of that yet.
With programming negotions and the locals push, HD has been put on the back burner for a while...
I think the HD stuff won't come up until May when TNT comes live, then some stuff may start to happen.