Because scheduling and organising for 8 people is very difficult to do
and also, casting that stuff is really hard, especially if good players are involved. The map is to big to cover all the action, so suddenly things will drastically change, and no one saw it happen! Only few casters can do it, and even then they are done talking for a week after only 1 match
that and that and 4v4s are boring to watch either.