Oberon was the first and most powerful Summer Knight. So puissant was he that he ruled alongside his Queen as an equal. Some say that he was a famous mythic figure taken up with a new name. Perhaps Gilgamesh, or clever Odysseus, or Prince Ōnamuji.
No one knows what happened exactly, but one day Oberon set down his sword and vanished as such legendary figures will do. All knights since then have been but poor, fragile replacements.
In another version of the story, Oberon was both the Summer and the Winter Knight and the Courts were joined in peace throguh the mutual love of the queens for him. But frankly this sounds a bit too much like somebody's harem fantasy, so not a lot of credence gets placed in that one.
The lesser fae are typically unhelpful on the matter, as it appears that they have been enchanted to forget.