This is one huge question with a lot of sidenotes attached to it. I will try to provide you a comprehensive overview on the matter.

First off, even among scholars and historians there isn't a totally shared consensus on which factors may be sought to define a society "feudal". Also there isn't a single school of thought regarding the effective duration of the "feudal society" in Europe. As a matter of fact, Jacques Le Goff, one of the most prominent and prolific historians of the past century, member of the so called "second generation" of the French Annales school of thought, postulated that the Middle Ages in Western Europe ended in 1789 with the French revolution, when the parliament of France formally abolished what they called "régime feodal". You may be more familiar with the notion that the Middle Ages ended in 1492, when the American continent was discovered, and that the rather short parenthesis of the Renaissance followed.
A recent scholar, Guido Ruggiero, in his book The Renaissance in Italy. A Social and Cultural History of the Rinascimento, claimed that the Renaissance would have started in 1260, the proposed year for the end of the Middle Ages.

If we want to analyze the "functional factors" of this society, which was not homogeneous throughout the continent and saw many peculiar instances of the same type of government, we must start from the very clear premise that it was a society where several aspects appear to overlap and at times weld together, in a structure mostly connected through personal links between people and not exactly within geographical spaces as we understand political entities, like for instance a modern border.
The basic "unit", if you will, of such system was not the average denizen of a farm or village, but rather the person who held said farm or village either as personal possession or as either a reward or loaned public good for the exercise of delegated authority. This subject we have called a "vassal", a term designating someone who is in a personal contract with a person of higher status and to whom he owes some obligations. In return for his fealty and loyalty, a plot of land, a village, a castle or a quota of taxes levied from another village or tariff applied to a certain traded good would be provided to this person. This is what is called in the period documents a beneficium, literally "benefit". Thus, a recent school of thought has started renaming the "feudal society" in "vassallatic-beneficiary system", as the concept of "feudalism" itself has undergone a period of rethinking and revision.

Now we enter the dangerous part.

One could argue that the fact that a society with little in the aspect of social tensions or conflict as we understand them to be, added with very few instances of education and low rates of literacy, limited movements along the social ladder and a general stratification of its classes may have contributed to the longevity of this system.
But we must also consider that these tendencies were hardly homogenous (possibly with the only exception of the literacy levels) during the very long span of the Middle Ages (even by its shortest value, it lasted 800 years, 1400 by its longest). Society during the IX century in France was quite different from the one during the 1100s in Northern Italy or the one in the late 1390s England. Social advancements would have been possible during the central centuries of the period if, say, a farmer chose to migrate to Florence in the 1180s and start working as a labourer in the wool industry, possibly seeing his son or grandson become the apprentice of a craftsman and own his own shop. Similarly, a very poor peasant could have his son join a monastery to become a monk or priest, possibly having him elected abbot or becoming a bishop (although this latter was more common from the XII century onwards).
It is virtually certain that people perceived their condition in a much more "total" way than we can understand. A goat herder would have never aspired in his life to become a knight or a count just as a knight didn't fancy setting up shop and becoming a tailor. But once again, even this was not always completely the case as we know that in XIII century Florence, the merchant and writer Dino Compagni, who was part of the council which ruled the city, felt uneasy thinking about the city's knights and aristocrats agitating political strife due to their aversion to merchants and bankers, which were as rich but didn't account as nobles or were perceived as such.

The political powers of this period were not stable either. I've mentioned the political strife in Florence, which often resulted in plain violence, feuding parties and families and civil war. Similarly, single lords and vassals were very often in war with one another in an attempt to curb someone else's power or to settle grudges and claims. Kingdoms, polities and ruling families were never fully sure of their independence and existence. Counts, lords, vassals: they all risked having their possessions revoked or their rights and privileges taken away; similar fears were shared by the cities.
The aforementioned political violence in XIII century Florence (orbiting around the support for the pope and the support for the emperor) at times meant risking losing influence to either side, like when pope Bonifacius VIII sent cardinal Matteo of Acquasparta to quell the feud between the banker families Cerchi and Donati, or when in 1301, again behind the influence of the pope, Charles of Valois moved to the city and expelled one of the feuding factions.

There were revolts. Not only the occasional disgruntled vassal. In the Middle Ages we see three huge rebellions with peasants and labourers at their core. All of these occurred during the XIV century, in France, England and Italy. There was the rebellion of Wat Tyler, also called the Peasants' Revolt of 1381, when the English peasants revolted against king Richard II asking for less taxes and the abolition of unpaid labour owed to the lord of the land they worked, as part of their contract of land renting. Some claim that Wat Tyler demanded the abolition of all noble and ecclesiastical privileges and a redistribution of all the lands in England. In the same year, in Florence, the masses of daily labourers working the bottom rung sectors of the wool industry revolted against unfair payment for their job and the prohibition to organize themselves in their own trade corporation as all other craftsmen did at the time. A few decades earlier, in 1358, French peasants revolted and murdered knights and nobles when the country was torn by war, taxes were increased due to the conflict and the unpaid forced labour was enforced for the construction and maintenance of their estates and castles.

After such a huge introduction, ultimately I believe there was no single factor which favored the longevity of the feudal society, just as I think there weren't for the length of the Roman or Greek societies. The Middle Ages had its fair share of contradictions: in the 1200s and 1300s you could have oscenely rich bankers and merchants moving goods and sums of money from Venice to Gand to Visby, buying and renting houses just as you had wealthy knights jousting against one another.
I do not agree with the Marxist historiography and perspective of historical evolution, as I believe that the concept of class strife was not a key factor. Neither was the relative wealth or juridical structure of said society, as well into the XVI century nobility privileges and rights were not only enforced, but also had become attached to a given title and fief, when monarchies were well into the transition towards a modern territorial State, rather than a collection of private subjects linked to a ruler through personal bonds. This process, ironically, was possible also thanks to the greater amount of wealth brought by the expansion of trade and city networks from the late 1100s onwards, permitting for a larger bureaucracy made up of lesser nobles or not noble at all, formal embassies and ambassadors and embryos of standing navies and armies, along with a centralized justice and a better system of tax collection and resources exploitation.

Possibly, the single factor we all link to the period and perhaps influenced it the most, was the weak or total absence of a centralized, higher authority for most of its duration, which could prevent regional powers from being born and/or reinforcing and enforcing themselves given the great influence held by the possession of landed wealth and the attraction of unarmed denizens of said lands, to be organized in a micro-polity held by the military efficacy of said private individual.

Sources:
Albertoni, Giuseppe. "Vassalli, feudi, feudalesimo". Carocci, Roma, 2015;
Persson, Karl Gunnar. "An Economic History of Europe, Knowledge, Institutions and Growth, 600 to the Present". Cambridge University Press 2010. (Italian edition of 2011)