The theory I remember from the dim and distant past of Usenet days (days of lore, way before even the start of Chrons - do we do time before Chrons, i.e. BC now? - Anyway I can't find the theory on today's internet, so now undoubtedly lost....) is that Terry Brooks read LotR, got a bit obsessed then came up with, in his eyes, a cracking sequel. Perhaps tentatively asked if he could attempt it - completely rebuffed by Tolkien and everyone else, so...had to write a LotR 'rip-off' that had enough differences from LotR, but enough similiarities to connect to this sequel idea.
That it also tapped into the market for Tolkien-esque epic fantasy I can believe too.