Unlike most locks, heel hooks dont control your opponents body enough. You can slowly apply an armbar or shoulder lock as you control their body until your partner feels pain and taps, you let go, no worries go again. A proper heel hook though needs to be applied a lot faster as it's a lot easier to escape, on top of that, there is not a lot of buffer between when they start to feel pain and an injury is imminent. So if you're waiting to feel pain when someone is cranking a heel hook, good chance you'll end up injured-especially inverted as people are waiting for pain in their ankle, next thing they know all their knee ligaments are blown.
Bjj is made as a safe way to practice self defence techniques where no one (should) get hurt, but gains you the ability to really hurt someone in a confrontation outside of the gym.
MMA is that confrontation outside the gym, it's all about trying to hurt your opponent. So a lot more techniques are allowed: Neck cranks, biceps crushes, heel hooks etc and of course striking.
