ABC/ESPN also have a "limited" number of games to broadcast. Would guess the undesirable non-conference games are ignored by ABC to focus on the conference ones.
"The new agreement, which began during the 2007-08 season, includes up to 41 televised Big Ten football games - up to 17 on ABC and up to 25 on ESPN or ESPN2. In addition, all regional afternoon football games aired on ABC will be aired by ESPN/ESPN2 in outer-markets, making these games nationally available."