Is because Disney knows that the fans will latch onto anything Lucas puts out and claim that it is G-Canon over the Disney Trilogy. If they were to take Lucas’s original treatments and make a graphic novel out of it; so many people would consider it true canon over the DT because it would be a true representation of Lucas’s vision.
Of course the DT fans don’t want this. They want to forget Lucas and don’t want Disney to give people anything that could be considered true G-Canon over the Sequel Trilogy.
Disney will never allow Lucas’s true vision of the Sequel Trilogy to see the light of day in any form because of this. That is a fact that makes me eternally sad as a Lucas fan; because I would love to see what he would have done with the concept of midichlorians; and how he would have executed Luke’s exile. Also I’ve heard rumors that a ‘Jedi Hunter’ was supposed to be in his version, and that sounds like something that would have been appealing. Alas, we will never see it because of whiny fans of the DT who would cry over there being something that is true G-Canon.