BET has been on Dish for a very long time. I actually have Thea tapes from when I had my 1000 and it used to send a IR signal to the VCR to record, so it definitely has been a while and certainly before MTV started all the new specialty channels. BET has become a showcase (dumping ground) for former UPN shows. Its focus has become young, hip, urban, black audiences (like UPN). And, we all know how UPN turned out. I seriously doubt if I were young, hip, black and urban that I would be spending Sat night watching Mo'nique - I'd think I would have more exciting things to do than watching TV. Therefore I doubt if its ratings are really stellar.
TVOne is what BET used to be - family entertainment featuring black families, which I think would appeal to more than just black audiences, with shows like the Jefferences, the Hugleys, Good Times, etc. I remember when BET used to show programs like What's Happening, the Jeffersons, Thea (my favorite), etc.
BTW, Mr. Ergen also once said that Dish was a family company and there would never be a place for adult channels on it - however, values met economics. So, I am not sure that the reason for TVOne's absence is duplication - look at the TLC family of networks. If Shalom in the Home can find its way on Green, you know there is a lot of recycling of shows, but we have nearly the full suite from TLC on Dish.