Ask yourself, Why isn't a conference championship followed by a showcase, bowl trip enough (anymore)?
The only answer I have: A large number of football fans can't live with ambiguity and ESPN is only to happy to feed the argument with more football programming.
I don't think it's the fans so much as it is the media and the conferences. The fans already bitch about too many trips and not enough time to get decent airfares, hotels, etc. Media, particularly ESPN, don't like the ambiguity. They feel an overwhelming need to crown somebody "champion." And for ESPN it's the added revenue from the extra game. The conferences want that added cash too, especially if one of them can have both teams playing for the title.
If there's a better alternative for ambiguity, I would have conference championships on Thanksgiving weekend; the regular season for conferences without divisions would also end that weekend. The playoffs (either 4 or 8 team) would then start at campus sites in the first week in December if there are 4 teams (first two weeks if 8 teams). Then the money New Years bowls could host the national championship on a rotating basis. Fewer trips for fans of the higher seeds at least, and being a top seed would mean something by having a home field advantage.