have their playoffs competing with NFL for fan interest and travel money.
Also, if you only have 8 teams in the playoff, you could still keep the bowl system in place and have some relatively strong "leftover" teams available to play in the bowls. The more ranked teams you cram into the playoffs, the less appealing the bowl system would be - to the point it might eventually disappear, and teams like BYU and Utah (in most years) would have no postseason at all.