Simply put, Dish signed a contract with SiriusXM, a while back (I don't know the exact date) and it was for the channels that existed then. Pop Rocks didn't exist when the contract was signed, so Dish can't retransmit the channel.
If you notice, SiriusXM replaced the Faction punk rock channel on channel 41 for Turbo (90s and 2000s hard rock), and moved Faction to 314, which is internet only, and was Turbo's old channel spot. Yet Dish still has the channel 314 version of Faction on channel 99-41, as Dish's contract with SiriusXM included Faction and not Turbo.
All those channels were never available at launch, and as I stated before, Dish had to update the contract to get those channels added. But, it isn't just Pop Rocks, and it isn't Dish choosing which channels they want. It's the signed agreement that only covers channels identified under the existing contract.Except.... DISH has other channels that were not around such as Pitbull and DISH has made other changes over the last few years. But Pop Rocks is a newer channel as you say so perhaps it's just that it was never added to DISH.