1. Time travel!
Everyone loves time travel. As long as you're not creating universe-rending paradoxes, time travel is a blast!
2. It's hilarious!
The games has a perfect mix of humor and intensity, which really brightens up the story. My personal fave: "This is a state of emergency! Kids should be home playing Nintendo games!"
3. Complex and unique RPG!
Earthbound literally revolutionized the RPG scene when it was first released. In a market monopolized by swordsmen and dragons, it showed the world that an RPG could be something different.
4. Deeeeeep symbolism.
I'm not going to go into detail, but some of the stuff the game throws at you will keep you thinking for weeks. It's nothing short of profound.
Although a hit in Japan, despite an aggressive marketing campaign, Earthbound failed to make a splash in the American market. It was released almost 15 years ago to rave reviews, and still makes an impression on those who choose to play it. So dig out your old SNES or boot up an emulator* and give it a try!
*I do not condone the use of emulators for illegal purposes.
I've been waiting since the release of the Wii for this to finally come to the Virtual Console. I doubt it will ever see release, though, because Nintendo's attorney aren't very happy with the way the game is full of sampled music from several American and British performers. The only way the game could come out (aside from firing the attorneys) would be from much of the music to be remade. I doubt they would spend the resources on that.