The emergence of powerful and independent towns is probably the main cause, for several reasons:
It provided taxable income, without which there can be no finance, thus no bureaucracy, thus no state and centralization. The demesnial economic organization which prevailed till then was, of course, agrarian, with market activities very limited in scope and quantity, and based primarily upon barter.
It resulted in the weakening of the nobility and the clergy, in two ways:
- First by questioning and challenging the seignieurial and ecclesiastic authority itself, by refusing to submit to it's encroachments.
- Second by providing to the king an ally against those social classes, thus resulting in a much more dynamic and fruitful class struggle. This cooperation was crystallized in what is known as the royal charter of liberty, which, as the name suggests, means a guarantee, by the king, of the town's freedom. By these contracts, the towns took upon themselves to furnish the Crown with tax money and foot soldiers. The following quote from Palmer & Colton provide a pretty good picture of why the charter was to so important to the townsmen:
"The merchants and craftsmen who lived in the towns did not wish to
remain, like the country people, subject to neighboring feudal lords.
At worst, the feudal lords regarded merchants as fat possessors of
ready money; they might hold them up on the road, plunder their mule
trains, collect tolls at river crossings, or extort cash by offering
"protection." At best, the most well-meaning feudal lord could not
supervise the affairs of merchants, for the feudal and customary law
knew nothing of commercial problems."
(If one reads Pirenne, one finds out that in Flanders for instance, the class struggle was much more complex than in other parts of Europe. The French king sided with the upper bourgeoisie against an alliance between some of the Flemish counts and the lower classes of the towns, who felt a deep resentment towards the ossification of patrician privilege.)
The following quote, also from P&C, give a pretty good summing up of what the consolidation of monarchies meant:
"They [the New Monarchs] especially enlisted the support of middle-class people in the towns, who were tired of the private wars and marauding habits of the feudal nobles. Townspeople were willing to let parliaments be dominated or even ignored by the king, for parliaments had proved too often to be strongholds of unruly barons, or had merely accentuated class conflict. The king, receiving money in taxes, was able to organize armies with which to control the nobles."
Although the above quote concerns rather the late than the high middle ages, the historical phenomenon is essentially the same: the Crown's need for liquid assets to control the nobles/clergy was supplied by the towns, whilst the towns' need for military protection to control the nobles/clergy was supplied by the Crown. The common denominator was always there: the need to control, to restrain, to contain the power of the nobility/Church. Thus was the pact assured.
L.E. I would also like to point out that, as the two historians also show, the reasons why Northern Italy, the Low Countries and Germany did not form into strong monarchies was that town life was actually so energetic and developed that it undermined not only the Church and the nobility, but also the Crown. The same was not true for France, England and Spain.
L.E.2 One must not imagine class conflict was always black-and-white. In France for instance, Louis VI and Louis VII worked hard with abbot Suger of St. Denis to emasculate the barons. They were practically ping-ponged between royal and ecclesiastic law until their prerogatives were seriously curbed. Also in France, but in a different context, the counts of Champagne were famously benevolent towards urban life, actively encouraging the great fairs of the region, which were unparalleled in Western Europe.