Let's see. It's because it's as good as any 2-D platformer ever made, and I don't give this sort of praise ever.
Why? It has simply has the greatest level design that I have ever seen for this type of game. I constantly asked myself how the heck they thought up all of the brilliant stuff in it. The variety is fantastic and everytime you think that you've seen it all, the game throws you a curveball.
Now people that say that the replayability is limited haven't played the game. After you get all of the Kong letters and puzzle pieces there is a mirror mode where you don't get the to use Diddy Kong's hover jump and you die from your first mistake. Then there is time attack mode where you can get bronze, silver, gold, and shiny gold medals. Try to get all of the shiny gold medals and see what happens. As Rol stated, only a few people have done it and it's nearly impossible. I'm going to try to do it, but don't expect to, and I can normally complete ANY game 100% with a decent struggle. I don't know if all of my gaming efforts will ever result in me getting all of the shiny golds.
Proud member of the SONIC SUPPORT SQUAD
Tag "Sorry man. Someone pissed in my Wheaties."
"There are like ten games a year that sell over a million units." High Voltage CEO - Eric Nofsinger