Fewer things are more frustrating for a medic than calling in a STEMI. First they have to send the ECG in for the physician to see before they arrive at the hospital, and then they are asked to stop at the door to get another one using the hospital’s ECG machine. When they could be out saving more lives, duplicating their efforts is the last thing they want to be doing.
There used to be a time when prehospital ECGs were lacking in quality compared to those obtained using hospital machinery. Field ECGs simply weren't of diagnostic quality - but today's field monitors have come a long way and are now of comparable quality to hospital machinery. As
such, it no longer makes sense to require medics to repeat their ECG upon hospital arrival.