The acting of the prequels simply wasn't on par with that of the originals, and that has nothing to do with a lack of talent (McGregor, Portman, Jackson, Neeson, etc.). It was largely rooted in subpar-to-terrible dialogue and directing, except for Christensen, who delivered one of the most wooden performances that I have ever seen. There's also something to be said about the cohesiveness of the original trilogy and overall better and more balanced production. Aside from that, 4-6 were groundbreaking films, a la Psycho, The Matrix, Jurassic Park, Avatar (I think Advent Children was more impressive.) that revolutionized cinema and defined cultural moments; the nostalgia is rooted in a bit more than chronology. I've literally seen 3,000+ films (probably more), and ROTJ is one of my favorite films of all time. While I enjoyed the prequels, they lack the emotionality (for many reasons) of the original trilogy.