I don't fully know, however, I will note that to my understanding, since Moses wrote the fist five books (Genesis, Exodus, Leviticus, Numbers, and Deuteronomy) if you will note... Moses came long after these events. I don't know what his source material was, but if it was word of mouth or other scrolls, its possible people in his time had access to those other scrolls which are now lost to time. If he felt that anyone could read the other scrolls for more information I could understand why there's not more information about these people.

I have no idea why some people do what they do. I will say I am very jealous of the Amish because they don't have the stresses I have, or half of the issues I have. No money for gas? I don't think they need to worry or care about it.

The other thing is, what does it really mean that he made a city? It could mean that he started an encampment elsewhere. we don't know how many other people God would have made during Adam / Cains time, I would imagine God would have made Cain a wife at some point.