Well for one, it's a very well designed and built game. You can tell a lot of time and energy went into it. It is simple, but as we all know, simple is sometimes the hardest thing to do. Pixel perfect design plus smart sounds helps it stand out from the crowd big time.
A great article in Tech Crunch is a sign betaworks wasn't just crossing their fingers and hoping that people would notice the app, they worked the marketing and PR channels hard (as every developer/company should before a launch).
Wether it was the quality of the app itself, the successful press, or both, the app ended up getting featured in New and Noteworthy in Apple's App Store. That's absolutely going to give you a huge boost. Certainly enough of a boost to get into the top charts. And from there, given the unique nature of the game and the solid UX (Value Proposition and Value Delivery) getting up to the very top of the charts ensures that millions of people saw the game and at least a million downloaded it :)
I hope that helps! If you (or anyone else) is looking to launch an app, feel free to reach out. I used to work at Apple in the App Store/iTunes so I know quite a bit about the process.