Again, a medieval economy is not an industrial one. In the 1800s, there was plenty of urban demand for food that rural farmers could cash in on, but in medieval western Europe we're looking at roughly a 97%/3% rural/urban divide. With that kind of imbalance the average rural farmer would not have the opportunity to sell off a significant amount of excess crop. The opportunities for an individual serf to profit off of increased productivity simply weren't there.

By the late medieval period, sure, there was somewhat more robust intercity trade. But that's approaching 1000 years after the fall of the western Roman empire, which is stretching it if you want to claim cause and effect.