The previous system was easier to gamble. You put up a new app, pay for ads/bots/whatever, get a plenty of downloads and start showing up on the top for all the search queries (even if your app is crap), this in turn causes even more sales.
Also, in the previous system you could've positioned for a term completely unrelated to your app (e.g. you have a weight tracker, and you position for celebrity phrases because they're more popular). My guess is they try to stop that.
Oh, and even in the past it wasn't just popularity that guided the results. We did a detailed analysis of this ( techcrunch.com/2012/03/12/appcod-es-launches-app-store-prediction-tool-tells-developers-which-keywords-work/ )
As a developer, I had a path to improve my ranking which was to find a way to increase my downloads. I agree, the keywords model is somewhat of a fail in general because people can pick unrelated words, but this new version is actually worse than that.
Also, in the previous system you could've positioned for a term completely unrelated to your app (e.g. you have a weight tracker, and you position for celebrity phrases because they're more popular). My guess is they try to stop that.
Oh, and even in the past it wasn't just popularity that guided the results. We did a detailed analysis of this ( techcrunch.com/2012/03/12/appcod-es-launches-app-store-prediction-tool-tells-developers-which-keywords-work/ )