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.

How to Reduce Ankle Pain While Running? Avoid Heel Striking!
Another reason heel strike running is self-defeating is because there are too many negative forces at work on the foot-ankle complex which a lot of ankle injuries stem from as compared with forefoot running.