It's a Nevada State Athletic Commission rule. You can easily cause a concussion, cause brain damage, cause paralysis, and cause organ failure with a well placed blow to the back of a skull. You have no opportunity to tap to avoid this.
NSAC rules also state that a fighter knocked unconscious cannot fight for at least 3 months and after a medical clearance. If an athlete is beaten unconscious, he's out of circulation for that time and not making promoters money.
Worst case scenario with that "gay ass leg lock" Mir put on Lesnar is ligament tears around the knee and perhaps broken shin bones. You may limp for the rest of your life if your heel is twisted hard, but you will still live a fairly normal life.