We modify NHL 2004 because it's superior in every single aspect (compared to other PC NHL games).
The ai is ridiculously superior.to anything else ever released on PC. NHL 05-09 (PC) are pure crap in comparison. Those newer ones are arcade games, really. It's like comparing Austin Allegro to a Jaguar E-Type or Ferrari Daytona.
Interface is html/JS which makes it easily accessible and you can add pretty much everything there. NHL 05-09 have console interfaces which can't be altered. They have almost no statistics at all.
Those are the main reasons, really.
Best game engine, best interface, best everything.