DirecTV was paying way less than Dish or Comcast etc were paying. Viacom was in a position where if they gave in to DirecTV, they'd almost certainly be pushed into lowering their fee from Dish and Comcast. But Viacom's channels had decreased in ratings since the last deal, so an increase wasn't justified either. Was kinda guaranteed to be a protracted fight.
ETA: now that i notice that Viacom threw in streaming rights to DirecTV, it makes even more sense.