Argument from silence like "well, probably God did tell them somehow" is not a good argument.
This is incredibly biased. Absence of evidence is not evidence of absence; indeed, that claim is a well-known fallacy, which you are deliberately invoking.
Fortunately, we have plenty of positive evidence as well. In fact, God is explicitly recorded as talking to Adam (Genesis 2-3), Eve (Genesis 3), Cain (Genesis 4), Noah (Genesis 7-9), Job (Job), and many others. The idea that the only information God communicated to men is exactly what is recorded is simply absurd, and we see Cain and Abel offering sacrifices, and Noah doesn't seem to need to be told about clean and unclean animals.
So... why is this information missing?
A better question to ask is why do we expect it to be present? When telling a story, you generally don't fill in background information your audience already knows. It seems quite clear that people before Moses knew God, and knew at least some of what he expected in terms of laws and/or religious practice.
Another consideration is that Moses is generally credited for writing Genesis. That isn't to say that Genesis is fiction; rather, Moses took prior sources and compiled them into what we have today. Given that this likely happened at about the same time Leviticus was written (or at least at a time when Moses knew that Leviticus would be written), it's entirely plausible that he would have considered rehashing laws, practices and customs predating Leviticus to be superfluous.
It's true that Scripture is intended for us, today, but it was also intended for its contemporary audience... and for pre-Incarnation Jews, explaining the need and reasons to offer sacrifices would be as gratuitous as explaining the need and reasons to consume food or beverage. Thus, to them it would not be surprising that sacrifices are mentioned only occasionally, just as eating and drinking are mentioned only occasionally. Nor would it be any more surprising for no reason to be stated than for every instance of eating or drinking to omit "because he was hungry" or "because she was thirsty"; that information would be self-evident to the reader... just as it should be self-evident to us because we have the context of all of Scripture from which to infer such things. (It should be noted that there are many places in which understanding Scripture requires background knowledge; Genesis is hardly unusual in this respect.)
At any rate, the idea that God had not yet revealed Himself is clearly nonsense. The argument that people prior to Moses could not have known information that is not recorded prior to Leviticus is simply, and overtly, fallacious. It makes no sense when Scripture is considered in its original context, and it makes no sense when considered in our context today. In both cases, the "missing" information is either well known, or trivially inferred, from other sources.
