Why did extensive trading routes not develop in feudal western Europe?

Answer 1

Trade routes effectively develop primarily in areas under unitary control and protection. This is because a unitary authority can proposed long road projects and secure peace over a large stretch of territory. We see this in the Roman Empire, the Arab Caliphate, and the Mongol Empire, all of which had a unitary authority over a large stretch of land, built a large network of roads or secured general paths of trade, and saw an upsurge in international trade result.

Feudalism creates exactly the opposite scenario. Small pieces of territory are each controlled by differing authorities. Massive road projects were not undertaken. No lord could protect any area outside of his fief and due to occasional skirmishes with neighboring fiefdoms, a lord would need to use most his knights to defend his land in general as opposed to his civilians. Feudalism also retarded the growth of cities since urban migration was less safe than under larger empires. Since cities are one of the primary drivers of trade, this further hampered the growth of trading routes.