I think it comes down to budget and how much money each character costs.
It was 2 or 3 years ago when Dwight told me about the Western Boxed Set idea he had. He said it would have included a horse. Then he said when he pitched the idea it was shot down. I said really? By who? And he said walk with me over here. So we walked over to Ryan and Dan. Dwight and I brought up the Western set and they BOTH seemed dead set against the idea. "Horses are expensive" they said. Dwight and I kept on them and they repeated it. "Horses are expensive."
Now at the SDCC Legends panel I believe it was Jesse who said Ryan was the one who was against horses and he wasn't with the team anymore. But my feeling is, whether it's Dan, Ryan or whoever is in charge, the price is the price. And if they don't have it in the budget, they're not gonna spend it.