Now, I haven't Final Fantasy XIII-2, but for XIII, I remember explicitly reading that things like towns had to be exempt due to the huge amount of time and effort it would take to render them properly. This would not be a burden if every generation, and Square in particular, had to push the envelope of 'next generation graffix!!11' every couple of years.
Hard to tell to be honest with you, while i do agree a simpler looking city might help a lot but i don't think it is the whole story here. I have been actively playing modern JRPGs in general what i think is the real problem here is importance of city/town in a traditional sense has been dropped a lot just like what happened to the Overworld. Today, I expect a town to function like a guild to take multiple quests or town is simply menu " Shop, Inn, blacksmith,etc" or point and click town map.