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 Causes Slipped Discs
When it comes to protecting your lower back long-term, the research reveals that foot strike matter immensely whereby forefoot running prevents the weakening of the shock-absorbing structures in the back, preventing impact-induced degenerative changes to the back that are caused by heel strike running.