I think as the average gamer age went up, the expectation of sim like sports games became more prevalent. And when companies got the sole right to make a league's sport, all the players were consolidated.
Just look at the numbers for Madden, 2k, Fifa, etc. They sell a crap load each year (with secondary games for soccer doing pretty good too, and a mix of results coming from other sports games like UFC, Wrestling, etc). It's just that less games are made.
We've had the resurgence of Blitz and arcade like sports games here and there, but I don't think people really want to buy that, hence the low numbers.
So I guess the answer isn't important. It's more about the question. What happened to Sports games fans? I just think they got old.