Not just DK, but Alderaan also has very few roads. My only theory here is that they invented speeders before the wheel
You joke, but that's literally what happens with these planets. When colonized, the people take all their knowledge of current tech, such as speeders, instead of starting from scratch and reinventing everything. So yeah, not much need for roads.
Even so, I do agree. I'd like to do more fighting in the underbelly of Dromund Kaas. There seems to be a great city in the backdrop but we never get to explore it, apart from the main square and the Citadel.