The reason for this: Luke's saber's core was an in-camera effect achieved by a reflective sort of tape thing on the blade. When they saw the effect in post, they decided it wasn't good enough, and added a glow around it.

Ben's and Vader's, on the other hand, were (mostly) rotoscoped from scratch, core and all, in post, like lightsabers were in the rest of the saga.

So, if anything, Luke's saber in Star Wars should be altered to match his Empire saber.

--edit--

Besides, I like the Empire sabers the best of all six movies. So admittedly, I'm biased.