Bare-knuckle boxing matches had no time limit and fighters were allowed to grapple and throw, the match ended if a man hit the ground. No matter how well you punched, if you were grabbled and thrown to the ground, you lost the match. Therefore, it was advantageous to stay away from the opponent and use straight punches thrown from a distance. Modern boxers wear gloves so a high, tight guard helps them since the gloves may be used to pick off punches. Without gloves, fists cannot be used to block a fist, so the most effective way to fight was to stay back and deflect or parry punches, which, along with the need to defend against grabbing, dictated a low, extended guard. As the rules of boxing changed, holding was eliminated, and gloves were introduced, the more effective guard for the new rules became a high, tight guard.
Competition in Taekwondo, Karate, and other striking martial arts is more similar to bare-knuckle boxing than to modern boxing. The competitors fight at a long range, allowing more time to react, long range kicks are used, and hand attacks are usually straight. To score more points, most attacks are to the larger and lower target, the body, so the guarding the head is not as important.
The martial arts were not originally developed by warriors for use on the battlefield or by athletes for use in competition; it was developed by the unarmed public as a way to defend themselves against attackers. They were not concerned with killing or scoring points, they merely wanted to be able to defend against attacks and stop their attackers from continuing their attacks. It may be assumed that since this was their original purpose that what they developed would be what was most effect for this purpose. However, they did not develop guards! Look at the old traditional patterns, not modern patterns, and you will not find guard positions. Since patterns were supposed to be the teachers of the arts, why did they not teach guards?