I'm really kinda surprised we haven't seen a gunboat already in Rebels. It would fit the show very well.
Budget limitations. The show probably only has the time and money for so many new models per season. GL poured a lot of his own money into The Clone Wars which meant they could afford a lot of new environments, ships, big sequences, etc. on a regular basis. Even if they aren't new ships, this season has had new models in the form of Ys, TIE/IN and now TIE/D - no idea how much of their resources it takes. I still hope they do it, just I also know we can't expect a rush of new things to appear right away one after the other.
Edited by UnitOmega

