Without having actually played the game, I would dare to say it's because it has the *CRAFT in it After it, people just found it addictive.
Now what I'm curious about is WHY the game took off. The concepts of it are similar to various other games that never really were successful, after all. This is interesting, though, I suppose Minecraft isn't as original as I thought it was.
Less cynical thoughts: From what I'm reading, the concept wasn't new, but it has been retouched and tweaked several times until it took off. Well, that's how good games are made: through iteration until you find the right formula. It's amazing how some tiny unsuspecting detail can make a whole difference. Don't forget the quality is often in the details.