... because the hospital wouldn't allow him into the ER unless he arrived by ambulance.
Source. The man had crashed his car into the hospital parking lot. Witnesses flagged down a couple of nearby police officers, one of whom started CPR, while the other ran into the ER, only to be told this:
"When the officer ran into the hospital and asked for help, he was told he needed to call an ambulance, Simpson said. The crash was approximately 100 yards from the hospital, but the victim could not be processed without following standard ambulance protocol."
...
"The officer performed CPR for six minutes while waiting for the ambulance to arrive. The announcement came several hours later that Marin-Fuentes had died."