This seems a bit of an error to me, because the next Market System upgrade (Marketplace) and the Roads upgrade that it requires (Cobbled Roads) – come together on Minor City level.
Because of this there seems to be a significant delay in ability to recruit Merchants (can only be recruited starting from Trading Post), because the growth of the settlements is incredibly slow and requirements for Large Town are quite big (6000 population, considering most settlements below or at 2000). I'd probably take over the whole map before there was any significant Merchant Trade presense.

P.S. Maybe this is a Wrath of the Norsemen submod exclusive error, don't really know.