their math is a little bit off, a copy sold every 3 seconds = 28,800 sales a day. Or 10.5 million copies a year. To tell the truth its pretty shocking how minecraft has taken off...the guy will probably end up making 100 million off the game. And if he sticks to iterating it, it might end up making over a billion over it's lifetime(say 40-50 years). Which may seem like a huge number, until you remember that World of…
Speaking as a someone who has never played either, it's also shocking because World of Warcraft looks like this: http://www.sopheon.com/Portals/1/WoW%20Screenshot.jpg While Minecraft looks like this: http://www.indiegames.com/blog/images/timw/minecraft.jpg
Minecraft, on the other side, is about you, your personality and creativity. You're able to modify the game as you like: First off all, you're making your own world! Second, texture packs are available, in order to change both the world and your own character. By making the game less complex, it is also very easy to get into, compared to WoW.
(I'm sure there are other reasons as well, but I'm no professional game developer.)