The decline of feudalism was influenced by many things some of those include..
The nobles in Europe were fed up with the king, because he stole their sons, lost the war with France, heavily taxed them, could do anything he wanted to, & could marry people off to another person without either of their consents. Overall King John was a bad king, so they created the Magna Carta. The Magna Carta was a charter that limited the kings power & gave more to the people. The Magna Carta shifted the power of Feudalism, helping it's downfall.
The Hundred Years war was a costly war, both in lives & in resources, so of course it let to the fall of feudalism. The knights & peasants were most of the people fighting, but they were supposed to be the groups with the most people. If you take away the foundation, the building falls apart, & the peasants & knights were the foundation to feudalism. After they were mostly gone feudalism wasn't really as helpful as it was before.