The Big 12 held its last championship game last December. As a result, the league did not have a game scheduled on "Championship Saturday," the final weekend of the season during which the conference championship contests are held.
As the Big 10 discovered in recent years, not playing that weekend hurts in the perception of the polls and decreases visibility at a time when every vote matters in determining BCS fates.
As a result, there was some discussion that the Big 12 would move the Texas-Texas A&M game, which has been held on the traditional date of Thanksgiving for the last three seasons. However, to avoid disrupting a tradition just restored, the conference opted to move the Texas-Baylor game in Waco from October 22nd to December 3rd, as well as moving the Bedlam game between Oklahoma and Oklahoma State to the same date.
So the conference achieves its objective of maintaining visibility (hence relevance) during Championship Saturday, while keeping the powers that be at both Texas and Texas A&M happy, as well as all of those hard-core traditionalists. An added benefit for the teams involved is another bye week late in the season so players can heal the various injuries incurred during the long conference season.