For me there a few reasons why I lost interest, a big one was sideline coaching, I know it's integral to xball but it just took a lot of the flair out of it.
I also agree with @
Dusty that there doesn't seem to be the characters about that make you watch, the arrogance of Martinez, Lasoya, Lundqvist and many others on the field, great guys off it but they just had an air about them that they knew they could do whatever it took to beat you.
Another big one for me was losing the mixed division games at the millenium, having only got to SPL level I used to love playing the one or two games at each leg against the pro teams, it made me take more of an interest, it would attract me back now as an old fart if I knew that by going to a millenium I'd get to play a couple of pro teams.