The big thing with Caravans seems to be war and Neighboring Factions/Towns. If you are in a Faction(say Vlandia) and at war with 1 or more neighbors, you severely hamper its progress as it; has to travel further, avoid enemy parties, Limits close buy sell options.
So either run Caravans when not in a faction or just from a blown up faction with lots of different neighbors. Vlandia etc bad, Empire Good.