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.

Heel Strike Running is the Main Cause of Overpronation
Heel strike running is one of the main causes of overpronation because the foot rollover process that occurs right after heel strike was found to force the heel into extreme positions, especially in shod (shoe) conditions as compared with barefoot running.