Lucas always liked doing that, just sitting on the SW trilogy and not releasing it, he refused to release it on bluray or hd-dvd in 2006 (when it was re-released on DVD that same year as we all remember), and for five years until 2011 by which time he could have had new transfers done as is the norm now for HD, but he didn't. So what was the point in waiting 5 years to release it in HD?
Disney should be keen to avoid getting the same reputation.