While he wasn't seen on camera, WWE's John Laurinaitis rushed down to ringside with the WWE trainers. The first time they tried to clean Cena off, he wasn't aware what they wanted to do and went into a babyface comeback on Randy Orton. They finally were able to clean him off and glue the cut shut after the following fall.
I am told WWE wanted the blood cleaned up immediately because Cena and Orton were scheduled to go into the crowd and they didn't want a health issue with a bloody wrestler among fans (it's one of the major sticking points of the Pennsylvania State Athletic Commission) and because they didn't want to give Linda McMahon's Senate race opposition any new ammunition.
Not knowing how long it would take to clean the cut and stop the bleeding, WWE stopped the clock momentarily during the bout - because the blood was an unplanned moment and they wanted it stopped ASAP.