Because Disney is stripping away 30+ years of star wars lore to replace it with their own when the could have just placed the new story line farther in the future than the books. But to keep cameos of the characters from the original trilogy they refused to do that and scrap 30+ years of cannon that was approved by Lucas himself. To see some pretty good points check out Matt Wilkins (On Youtube.) rebuttals to people who disagree with EU.