Now, I am going to caveat by saying that I have deep sympathies for Elizabeth A.R. Brown's thesis that "feudalism" never really existed, but I think that what this question really wants to get at is why the power relationship between kings and the landed aristocracy shifted towards the former. It's important to note that some aspects of the medieval period (serfdom, free cities, etc.) survived long into the Early Modern Period.

First, the advent of more powerful siege artillery and the regulation of castle construction (in England known as the "right of crenelation"). Castles were a major force multiplier and gave even minor lords the ability to resist royal attempts to impose their authority on them. A very small force could hold off a much larger army, and the constraints of medieval campaigning (the opinion of other vassals, the need to maintain an army away from the center of royal power, etc.) meant that it was unlikely for the king to be able to maintain the siege long enough to force a capitulation. The primary means of asserting royal authority in the hey-day of castles was to use other vassals to oppress the unruly vassal, but, naturally, no vassal wants the king to have too much power. Siege artillery (first trebuchets, later bronze and iron cannons) changed this dynamic by making it possible for an army to quickly dismantle castle walls and removed the advantage this gave to feudal lords. More importantly, the creation of a formal legal system that constrained the powers of the nobility in the thirteenth century helped the kings corral important landholders and impose royal authority on areas far from the capital.

The second is the creation of professional standing armies. Prior to the Late Middle Ages, few states in Europe maintained standing professional armies. Most medieval armies were composed of vassal forces. Many armies relied upon the complex vassal ties to gather relatively professional groups of men-at-arms and knights. That said, these knights were more loyal to their immediate liege lord than to the king or any nebulous notion of "the nation", which is why areas like the Duchy of Burgundy in France were able to operate essentially independently of the nominal central government in Paris. Professional armies did away with these levels of authority by placing military power directly in the hands of the king, freeing them from having to appease their vassals to maintain their military strength. Only kings had access to the funds needed to maintain the cutting-edge military needed to stay relevant on the European battlefield, although many aristocrats maintained their own military formations and regiments out of their own funds, another drain on their resources.

Third, there were major economic changes happening at the time. The so-called Price Revolution in the 16th century and the advent of strong, international banking networks in the late Middle Ages allowed kings to amass much greater amounts of wealth than their vassals. Manufacturing in cities helped strengthen the power of the urban merchants against the landed aristocracy, a competition that helped the kings prevent either group from gaining too much power and forced them to rely upon the king for arbitration.

With all that out of the way, the Early Modern Period did not see the disappearance of the landed aristocracy nor the abolition of their power (as recently as the 1990s, the House of Lords in the United Kingdom was unelected and composed primarily of the hereditary nobility). In fact, the landed aristocracy often resisted attempts to impose royal control over their affairs. The French Wars of Religion, the Dutch Revolt, and even the Thirty Years' War all had their origins in attempts by the nobility to resist royal authority, though they all took on religious elements as well.

I'm going to add that sometimes feudalism staged comebacks of sorts. The balance and power shifted amongst the monarchy and nobility usually. And for want of a better term by the Renaissance period kings were able to consolidate fiscal might and thus military power to distance themselves from their major magnates. While quite often the nobility turned into something of a service nobility they expect to be paid too. In the mediaeval period this mostly consisted of land grants, temporary or permanent though the former tended to turn into the latter. A bad ruler could easily dismantle their realm due to bad fiscal policy and wasteful spending, wars mostly. In mediaeval Scandinavia both Sweden and Denmark at various points were almost entirely mortgaged this way. Denmark in one case effectively ceased to exist for a number of years in the 1300s as two bad kings had wasted the treasury and loaned money with fiefs as collateral. A particularly bad strategy as often you lost the income from the fiefs meaning the king found it ever harder to repay the loans and then loaned even more...

Now giving away lands as gifts to favourites or as rewards was common. And this tended to continue. Swedish history recognizes the period after the 30 Year War as one where the monarchy eviscerated it's own base of income. During the war there were problems paying leading officers, usually nobles, whereas mercenaries would expect cold hard cash you could give land to nobles in lieu of monetary rewards. And although Sweden used a levy based system it was difficult and expensive to provide horsemen so they gave taxbreaks to any household that provided a cavalryman for the crown. In more or less a corollary to the original "feudalism". Many independent peasants of not great means would take advantage.

Which leads us to a curious situation where the nobility had managed to greatly expand their lands and wealth, and influence at the expense of the central authority. If that authority is weakened, a long minority e.g. the nobility can push their positions forwards substantially. In Swedish history the pendulum shifts again back as stronger monarchs are able to cow the nobility and in several big revisions return estates the nobility has "unlawfully" amassed. Not in the least as they could lean on the 3rd and 4th estates.