It's because RotJ is pretty terrible when it's not outright amazing. The "Special Edition" only made it worse. It's strengths are the battle on Jaba's sail barge, the speeder chase on Endor, the entire space battle sequence, the final Vader/Luke duel before the Emperor, and Vader's return to the Light side of the Force. There are other singular moments I love in that film; Leia's attempt to rescue Han, Chewie hijacking the AT-ST, and Leia reversing the "I love you."-"I know." bit with Han. But beyond these parts, the film is painful to watch at times. I still think that it's mostly good, but by far the lesser film in the Original Trilogy.