Question: when are land owners more likely to petition the ruler to restrict the freedom of peasants to move around: when there are lots of peasants or when there are only few? That was the question asked by Professor Domar of MIT.
When peasants are numerous, their wages will be low, or subsistence. If you have to hire them you pay them little, if you enslave them, you still have to feed them. So slavery doesn’t make all that much sense.
The story is very different when there are only few peasants (relative to land). Their wages would be high and you would save yourself substantial resources if you could get the ruler to force the peasants to work for you for free.
In 17th century Russia, Ukraine and southern lands became open for settlement. Because of all that land, peasants’ options improved and the land owners petitioned the Tsar to restrict their freedoms (among others, the Law of 1658 made flight from their assigned estate a criminal offense).
In 17th century America, the colonial administration first tried to enslave the Indians, which didn’t work, and then tried to use indenture d servants to run the economy, which didn’t work because of the abundance of land. Bringing slaves from Africa became a necessity.