One physical reason that feudalism declined and royal authority increased is the longbow.

More modern than composite bow, the longbow was probably invented in 12th-century Wales. It was about two metres in length, and made of the highly tensile staves of the yew tree. The longbow could pierce armor at a hundred metres. This meant that a heavily-armored, mounted knight (who had to be trained some 5-10 years at great expense) could be beaten by a peasant with a strong arm and six weeks' training.

The English kings were first to utilize this weapon on a large scale. They saw the advantage in rejecting the traditional military method of using armies brought by feudal lords. Instead, kings allied themselves with the growing cities, taxed them, protected them from the agrarian, feudal lords, and used the taxes to hire mercenaries to use the longbow.

In terms of a defense budget, this arrangement worked much more efficiently than relying on aristocrats, the natural competitors of kings. For a tiny investment, kings freed themselves of feudal lords, and were now in a military position to challenge to power of these lords with mercenary archers armed with longbows. This process did not happen overnight (but it did put kings over-the-knights) -- it took a couple centuries to complete.

Knights and country nobles did not adapt even that quickly. They remained ensconced in their estates, and in a way of life that was dying out. Perhaps it was pride, isolationism, conservatism, or the feudal psychology of remaining free 'lords of their own castles' that hindered them from seeing that technology and social relationships were passing them by. They remained in a kind of Ga-Ga Land, romanticizing days-gone-by in tournaments and in the fading glitter of the castles' Great Halls, reliving the past when knights and petty feudal lords had a great deal of power and influence.

Reference: Johan Huizinga: The Waning of the Middle Ages.