Hi, Mackenzie.

That’s a good question. Like much of what appears in the Tractatus, the ontology of a “fact” seems to be simply posited as if it is self-evident. It is axiomatic, so to speak. Indeed, the first two sentences of the Tractatus are, “The world is everything that is the case.” and “The world is the totality of facts, not things.”

I think, though, while axiomatic and not well elucidated in terms of reasoning, I don't think his decision on what a fact is unfounded. A fact, being a way in which things are, seems to be a quite astute definition of a fact. For instance, it is a fact that I have blonde hair because I indeed do have blonde hair. It is a fact because it is a statement that corresponds with reality. So the statement, “Daniel has blonde hair” is one which is a fact, because it is indeed the case that, in reality, I have blonde hair. Conversely, the statement “Daniel has purple hair” is not a fact because I do not have purple hair — that I don’t have purple hair, is evidenced by the reality of the state of affairs regarding the pigment of my head.

Hope this helps. :)