Epistemic comes from the Greek epistḗmē (ἐπιστήμη), meaning 'knowledge' or 'understanding'. It's used primarily in philosophy (including the field, Epistemology, that bears its name) as an adjective meaning "related to the abstract concept of knowledge or knowability".
An epistemic limitation, then, is a limitation relating to knowledge or knowability. An individual epistemic limitation would be a limitation of the ability of a given individual to know things, and a collective epistemic limitation is a limit on the knowledge of a society, or of all people.
A full discussion of the Black Swan Theory is beyond the scope of this answer. It is sufficient for our purposes to know that it deals with events that are very difficult or impossible to predict. The sentence is saying that its subject arises because of the limits of what individuals and societies know or can know, and that this is why it is difficult to predict.