This gameplayer article looks into some interesting changes in the backend of the way the Rock Band series is changing and shows arguments as to why EA may have lost the series.
"Rock Band has undersold. For a game that is far and away a superior package than that of main rival Guitar Hero – better songs, better DLC, better drums, song compatibility across games - it sold half as many copies in 2008. We're not saying the game has sold badly – far from it - just that it has failed to eat into the Guitar Hero series as much as it should of. Why?"