Maybe because they know no one will like the mode enough to play more than 30 seasons?
Hell, most gamers....even diehard fans....probably cant stand more than 5 in its current state. By that point, I imagine the poor CPU offseason AI and CPU team management will have drove some people mad.
Seriously, I agree with your point.....but they need to focus on getting to a point where more people might wanna even get to 30. If they can make the mode that engrossing....then look into expanding.