Because... Look! A 3-headed monkey! [spoiler]I think having Rey and Kylo half-siblings works well with the arc and themes they've got going so far. Rey could be Luke's kid and you'd still wind up with that setup, but it seems a smidgen on the nose. [/spoiler] I dunno. I'm not a StarWars guy. I'll leave the conspiracy…
I'd say the fact that the movie hits pretty much the same story beats as a new hope is a very deliberate attempt to banish the prequels from people's minds as well as remind people and get others up to speed with a movie that came out a long, long time ago. I think the probably overdid it, really, but I can understand why…