Columbia could have easily chased down the gap but no other team wanted to help them. They left the onus on the other teams to chase it down. The other teams didn't take it.
Why is Cav unbeatable? Every sprinter knows they can't beat him. This could have easily been a sprint finish. Columbia didn't work because no one else would. They know where they stand. They now know that no other sprinter wants to go up against Cav.