You gotta admit, the guy who designed the game is genius. It has infinite replay value, people are still playing it widely to this day, not many games from the 80's can say the same!
I remember seeing it in the arcades when it came out in the 80's, and wondering what was up with this strange Russian game? There was always someone pumping quarters into it, though.
I think the success comes from the fact that pretty much anyone of any age can grasp the concept immediately, without any preparation or previous video game skill.