Whenever people talk about British kings, or list them, it always starts with William I in 1066. I'd always assumed that before him, there were a bunch of petty kings and he was the first to conquer them and unify England, but I found out that it was actually Æthelstan who had done that, over 100 years earlier. So why doesn't English monarchical history start with him?
I'm not sure what histories you are talking about, while the Norman Conquest was a significant event, I don't know why anyone would start a history of monarchy in Britain with William the Bastard.
You are right that Æthelstan unified England, but prior to that there had been kings of Wessex, and High Kings (Bretwalda) of Mercia and Wessex that dominated England during the Heptarchy.