Another possible factor could be the fact that doctors tend to make more mistakes during operations on Fridays compared to start of week.
STEMI centers (this is picked up by EMS and these ambulances are redirected to appropriate centers) have 24/7 cath lab coverage and any major one will have an ER bypass even during afterhours to expedite care.
In fact more and more hospitals (and all the major ones) announce a “Code STEMI” overhead either when the ambulance is dispatched or as soon as the ECG showing ST elevations is discovered in triage/ER to activate the team and reduce door-to-balloon time.