I'll just remind people of what happened when the BCS was started. People loved the idea that we would see a 1v2, but because it was in its infancy, people saw that all you had to do was play an easy schedule, but if you ran it, you could still be in the championship game. If you are going to take the top 8 teams, why would anyone schedule a tough OOC opponent? Why risk it when instead, they could just face 4 creampuffs, get to the top 8 and make the playoffs guaranteed?
If you start letting in too many teams, there is no need to try to separate from the pack. Just play it safe, schedule FCS or nobodies, get 1-2 losses in your conference and if you were ranked high enough preseason, you are getting in the playoff. Letting in 8 teams is where you start making it so that teams don't need to schedule anyone to get into the top 8, especially if they are a top tier team that starts the season in the top 10.
On the other hand, if you take the top 4 conference champions, teams not only have to win their conference (1-2 losses in conference could cost you that), but also, they need to make sure they are in the top 4. They need a tough schedule to make sure the computers or pollsters don't keep them out of the playoffs. Maybe a big win OOC can make up for a loss in conference in the eyes of the voters, so teams would be more willing to schedule someone.