The serf system rose from the way the Germanic tribes operated.
Since they didn't make slaves of themselves, they didn't bother with slavery.
In the 4th and 5th century, as western civilization collasped, people still needed food and safety, so at first, they relied on their barbarian kings, but these kings controlled such vast lands (with little communication) they needed men to protect and hold areas, so they gave land grants to Barons, who in turn gave grants from their land to others, and so on.
Poor people, with nothing and nowhere to turn, arrived on these lands, and the lords would offer them protection if they agreed to work the land for six days a week, the 7th they could keep whatever they did for themselves.
At first, people were free to come and go, but there was no place to go, the great cities of Rome were deserted, as no food was to be had in them.
Over time, people forgot the cities, and settled into manor life, and accepted the fuedal system, the manor lord provided for all their needs, he provided a mill for flour, a keep for protection, artisans to make furniture, and so on, so the manor became a little set community, which the Feudal lord protected.
The system always put the landsmen at the bottom, and no matter how hard he worked, he always ended up oweing the Lord, so in time this debt became hereditary, and thus, serfdom.