My guess is that they wanted to redo the original 3 movies, to follow the 3 new (epi 1-3) movies, using today's animation/technology, etc. This would have multiple negatives, like having to recast iconic people like Harrison Ford, Mark Hamill, etc. Combine that with any deviation from the original "holy" three storyline/events/interpretations and you'd have a Star Wars civil war level backlash on your hands. I'm sure they didn't want to chance that. So a way of keeping all the original cast, leave the original stories how they are, and even better, bring them all together, but use those scripts to a certain amount by making them parallel stories makes everyone happy.
I don't have any issue with the parallels, it actually fits perfectly with the original methodology behind Lucas's story being based on Joseph Campbell's Hero With a Thousand Faces. If you aren't familiar with this idea, I suggest you check it out. I took a class in college that talked a bunch about this and found it pretty neat, even if you're not a Star Wars fan, but especially if you are. Once you see it, you'll start to see it in pretty much every good "hero" movie.