Really good question. I haven't seen something stay on the top spot of the front page for this long in a while. 1. It naturally has a great difficulty curve.

Really good question. I haven't seen something stay on the top spot of the front page for this long in a while. 1. It naturally has a great difficulty curve. 2. It feeds right into the kind of logic and algorithms that people on hacker news find so intriguing. 3. It's very easy to pick up how to play and share with others.