Heel strike running is bad because multiple lines of research over the last few decades have proved the many verifiable aspects of heel strike running that are primarily responsible for nearly all running injuries vs forefoot running.
Why Ground Contact is Longer in Heel Strike Running
Ground contact is longer in heel strike running than forefoot running because of 2 factors: 1. a posterior position of the center mass at foot strike and 2. foot rollover causes the foot to spend more time on the ground.