Maybe it's because the great John Williams, from the Imperial March to the Han and Leia Love Theme, created a score that is not only the oxygen of this universe but a constant manipulator of our heartstrings even today.
Maybe it's because of the Millennium Falcon encapsulates cool, freedom, adventure, and hope with every bank turn and asteroid avoiding barrel roll.
Maybe it's because Jim Henson and Frank Oz, two men responsible for bringing to life a majority of our imaginary friends for decades, brought to life yet another creature with more pathos and iconography than many seasoned actors...all while speaking in some strange form of pig Latin.
Maybe it's because Darth Vader, one of the top 3 best cinematic villains of all time, is more villainous and oddly clever in Empire than he is in any other film in the series...but still manages to make us feel for him at the end?
Maybe it's because the romance between Han and Leia was unconventional, confrontational, mean spirited...and still one of the best, most heartfelt and tragic couplings in cinema history...especially since it was originally supposed to end with Han dying.
Maybe it's because it really took the time it did on the self-actualization and development of Luke Skywalker from a meek farm-boy to a confident and capable Jedi upstart...something that would almost never happen today.
Maybe it's because it had one of the most jaw dropping, franchise-altering, twists of all time during a time where twists weren't a cliched Hollywood trope.
Maybe it's because the camaraderie and love felt between Han and Luke and Leia and Chewie and the Droids didn't seem like some manufactured thing for a movie. It felt obviously genuine. So genuine, you didn't need to hear Han and Luke say it when they spoke to each other for the last time in that icy Hoth hangar bay
You didn't need to understand the yelping of Chewie when he took those wookiee arms of his, primarily used for tearing off droid arms, and snatched Luke up in a hug.
You didn't need to hear Han tell Leia "I love you too" before he was frozen in carbonite. We knew just like he did.
To be honest, it's all those things and more. The Empire Strikes Back is the bar. It is the very APEX of the Star Wars universe and countless childhoods. It is what every sci-fi adventure film has aspired to be as good as...but not presume to ever be better than. Because there is nothing that will eclipse The Godfather...nothing that will eclipse Psycho...nothing that will eclipse The Shawshank Redemption...and NOTHING that will eclipse The Empire Strikes Back. It will forever hold a place in our heart. AND I DIDN'T EVEN MENTION HOW AWESOME LANDO IS!!!!