Question

Why were medieval manors nearly self-sufficient?

Answer 1

The model on which medieval manors was based was feudalism. This meant that lords gave land and protection in exchange for goods produced on those lands by the peasants. This gave a lot of bigger manors the access to self-sufficiency, it wasn't however always the case.

Answer 2

They were nearly self-sufficient because the wealthy manor owners would have access to farmland and food thanks to their workers, and they would also have an army to defend them in case someone attacked. They were essentially like small states where everything was organized.