Not a huge surprise, since no presidential candidate is about to be caught crossing a picket line at CBS Television City, where the Dec. 10 debate was to have taken place. Writers Guild members have been picketing that site pretty regularly. Another complication: a threatened strike by CBS News writers (also WGA members but under a separate contract). From the NYT:

The debate on Dec. 10 was never scheduled to run on the entire CBS network, as individual stations would have been able to choose whether to broadcast it or not. But the 90-minute debate would have taken place from 5 p.m. to 6:30 p.m. Pacific time, putting the Democratic tussle into prime time in the East and Midwest and making it unlikely that many stations in the Eastern or Central time zones would have chosen to carry the debate. It would have been available on CBS’s Web site.