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 Heel Strike Running Causes ITBS vs Forefoot Running
Heel strike running was proven to yet again cause another injury, ITBS, by causing the foot to wrestle too much with the ground during the heel-to-toe rollover action. This was found to transfer increases in bending and rotational stress from the foot to the IT band as compared with forefoot striking, which was found to have the most stabilizing effects on the foot-ankle complex throughout all phases of running gait.