Nascar won't throw a caution on the last lap unless the track is completely blocked. The leader had taken the white flag so it was going to play out. Had it happened 2 laps sooner,they would have bunched the field for a heckuva green-white-checker, but on the last lap they are not going to throw the caution. Despite naysayers,they are actually pretty consistent about it. The end of the race is managed differently than the middle.