Because it'll ding revenue from early-season games, especially out-of-conference games, which will now mean precisely squat to the season. College football is unique in that every single week matters. Go to 8 teams in a playoff, and I think that'll no longer be true. We could even see teams who clinch their divisions rest starters like in the NFL. That would be a bad thing for college football, IMO.
4 teams was a good way to get out of the BCS quagmire without going so far as to render the regular season inert. Being left out at #5 is a lot more palatable than being left out at #3. The only real problem with the 4-team format is that it needs a rule stipulating conference champions only.