I think they did a great marketing job. They released in December (good timing for Christmas) at the cheapest price point. Probably great timing to help it int

I think they did a great marketing job. They released in December (good timing for Christmas) at the cheapest price point. Probably great timing to help it into the top 10 initially. Then they made sure they looked after their users by consistently giving new levels for free for the first 6-12 months. Heck they are still giving free updates now! People love that, in turn the more users took the time to give it good reviews which then helped sell to more people & keep it in the top ten.