The short answer is this: Rules.
Okinawan Karate was created to allow an unarmed (or poorly armed) peasant defeat an armed and armored soldier/samurai, and as such, any attack that works is allowed, and even encouraged. If you're fighting in a resistance movement, you aren't going to lose much sleep over a dead oppressor.
With modern sport Karate, however, you really don't want to kill your opponent. You just want to defeat him, either by score or by knockout. If you kill or seriously injure him, you might get in trouble with the law or banned from competition. Also, if competitors kept getting killed or injured, people would stop competing. It wouldn't be worth the risk. So there are rules to prevent injury or death in sport Karate.