During the medieval period, the kings would establish a feudal system, which encouraged the establishment of serfdom as agricultural labor. Feudalism and serfdom were established in many parts of Europe, like Germany, France and England, but never fully in Northern Europe. Why was this the case?
We have no answer to the question, so we have to guess and have a discussion about it. At first it should be said that some Scandinavians lived in conditions somewhat similar to peasants on the continent. A recent book about Swedish colonisation of Estonia, says that the peasants had no say in whether to go or not. The owners of the land - in this case, the monastery of Roma, decided, together with royalty in Denmark and Sweden.
In general, there is a tremendous difference between central areas, that could have had a saturated population density already in the Neolithicum, and internal colonisation areas. Sweden did not reach maximum farmland until some 100-150 years ago.
Maybe this tells us something about slavery in Scandinavia. Given that there was possibly somewhere, where a slave could go and be free, must have meant that the slaves could not be pushed very hard.
If we imagine an increase in technology, making it possible for persons to escape to free lands, and the increased value of workers, because of the black death, it is not so far-fetched to believe that Scandinavians could live a somewhat freer life.
Even so there was at the beginning an obvious difference between different classes of peasants, one class are called landbor, in Swedish. They did not own their own land, and some of their ancestors might well have been slaves. As the high middle ages wore on - however - they were given their own farms, but that the land was still owned by a lord of some sort. You can see the spread of them in the existence of many medieval place names, meaning, new farms popped up at the fringes of land that had been cultivated for a long time.
Those peasants had to pay rent for their right to use the land. If I remember correctly, the contracts could sometimes be very long. If they were to be paid in some kind of currency, it is quite possible that, at the end, the rent was fairly small.
But, in short, increased technology, a shortage of humans, and... huge tracts of unused land, probably gave the poorest an increased leverage.
One indication of that which is discussed, is that as late as 1317, the Swedish king sent a tax collector - Lars Karlsson - north, but he never made it back. Such facts made it more interesting for the crown to strike deals, in which taxes were not so heavy, in places where the crown did not have a lot of power, also implying that such were to which peasants could escape.
Another - not certain fact - is that military power often rested on peasant levies more than - what they did - on the continent. If we look at some medieval Swedish laws, it even says that those in the levy should have - folkvapen - which is both sword, spear, armour and bow. It is possible that such laws made it more difficult for the elite to press an average peasantry into serfdom. Such thoughts might possibly find some support in the later organisation of Sweden. The enormous power growth of Sweden must in some parts be based on its ability to tap into a large part of the population. But that is not the answer for this question, rather, this question might be some of the basis for why Sweden could develop as it did.