Because the SE recomposited all of the individual elements for a lot of effects shots, especially on Hoth. Sometimes things weren't lined up exactly the way the were the first time. The Despecialized versions make a point of going back to the original shots if anything at all was changed in them. Not just reversing added new CG stuff, but also relocated/timed elements. If there's a difference, back to the original it goes. The original composites WERE blurrier. That's just part of having it (almost) all-original.