Each month, millions of people are getting an iOS device and opening the App Store for the first time and are unfamiliar with the apps that were popular 6 months ago, let alone 1-2 years ago. Doodle Jump is popular but it's actually extremely similar to Super PapiJump, an iPhone game that was huge a few years ago. Same exact mechanics, slightly different graphics & execution.
The mentality that you need an original, unique idea to succeed in the App Store is rubbish. A common algorithm for succeeding in the App Store is take Popular App A from 2009 and re-implement it with new graphics and perhaps an additional gameplay mechanic and release it as Hopeful Popular App B in 2011. Apps that were entertaining and popular (but have fallen off the charts due to lack of updates) were based on game mechanics that are probably still alluring to new audiences in 2011.