I could be wrong but I think a large portion of the problem is how EA (and Take Two) have handled their sports franchises in the current generation. Like most publishers they focused their efforts towards the XBox 360 and PS3, and (essentially) ignored the Wii. The end result has been that these companies have produce a sports gamer's dream on the two platforms which are not selling particularly well (so games see good but not great sales on these platforms), and have produced a sports gamer's nightmare on the system that has been selling really well (making for pretty bad but not awful sales).
Until the sports crowd (as a whole) can afford a PS3 or XBox 360, or the companies start producing every sports game at a decent level of quality for the Wii, the sales of sports games will not grow dramatically.
