Because the speakers need to be evenly distributed around you in a circle. In a room that size, the couch often ends up against a wall with the TV on the opposite wall (not sure how the 25 foot depth plays into this). Look here for advice on speaker placement and setup:
If your couch does have to be against a wall, then you might consider a 5.1 Onkyo system because the rear speakers need some space to really work.
I am very interested in this onkyo system after reading all the reviews. But one concern for me is the room size. My living room is 25 ft x 12.5 ft, but the area for home entertainment is 10 ft x 12.5 ft. I don't know if this room size would be enough to fit this bulky home theater system.
I would greatly appreciate your sugggestions!!! Thanks a lot.