Neither Dish or DTV is all that accomodating to ex-subs. Dish used to have a policy that a customer only qualified for one "new sub" deal in a lifetime. I've heard they are a bit better than that now, if you contact the right department, but even then, it takes a 2-3 year hiatus to get a decent deal.
If you bolted Dish for VOOM and then tried to come back within 12-18 months, then it isn't a surprise if they didn't offer you any kind of deal.
The reason for these policies is that if people keep switching to DTV to Cable to Dish and then around the circle again, over and over, and each time getting a new customer deal, the total cost to the providers would run into the billions.
So if you were on Dish and jumped to VOOM, but had never been on DTV, then only DTV was going to offer you a big deal. Or perhaps your cable company if you hadn't been with them before.