I think putting in a smoke unit would have required a rearranging inside the GP.
As Lionel's CAD drawing shows, there's not a lot of room to work with. The motor would need to be moved to the rear truck, and the electronics's moved forward to make room. Given what I see here, it seems like Lionel has simply modified the existing frame casting rather that redesign and retool it, which would add to the cost.
Rusty