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Because it's easy for NBC to show three to four dives,then five minutes of commercials and promos of upcoming shows on NBC after the Olympics are over,come back and show three to four more dives,then run five minutes of commercials,promos of upcoming shows on NBC after the Olympics are over and then back to Bob Costas promoting the next sport coming up,then two to three dives,then five minutes of commercials,promos of more NBC shows that nobody will be watching after the Olympics are over and then having NBC run a promo of an interview on the Today show for tomorrow about a USA gold medal winner who's event is coming up five minutes after the Today show promo is aired.
The best thing for NBC is that you can substitute gymnastics,swimming and track and field in the place of diving and you have the same thing and that is why,along with the time difference,that these most watched events are tape delayed for prime time.