Fifa used to be boring ping pong soccer, but that has changed. I think EA really took a risk when they made it more of a simulation - but it paid off (see 92 on metacritic). Essentially, they are now beating PES at their own game. They've also totally revived their NHL series. I don't really like EA, but they have done excellent work on these two games in recent years. They've been less progressive with Madden and Tiger Woods though.
I take the point though that they have focussed on consoles and neglected the pc version of fifa.
How much the quality of the game has to do with the sales is another question though. I suspect anual sales are much rather driven by marketting and the need to play with up to date rosters etc.
