Geez, I don't know, but I know it's not JUST that, because there are shots from the Elstree shoot that exhibit the same quality. The film really takes a large step in quality when Gil Taylor stopped using filters to soften and diffuse the photography (behind the back of GL, and at the behest of the studio, which pissed GL off, apparently). You can instantly see it - shots of C3PO and R2 hiding in that little alcove looking at the Falcon - you can instantly see how much sharper and clearer those shots are.
_Mike