Because it's an old version with much worse picture quality. Harmy's working (or will be working soon, can't remember) on a new one, with Blu-ray as source and a lot of tracks.
For ROTJ you should just wait for v2.0 IMO, but if you want to watch it now, the 7GB version from TPB is fine, there was no 20GB MKV of ROTJ yet.