Why did early boxers use vertical punches instead of horizontal punches? One reason was because they were taught to punch that way. People naturally punch with a horizontal fist; they have to be taught to punch vertically.
Who taught the early boxers the vertical punch? They were taught by trainers who were taught to use the vertical punch by their instructors. Just as in the martial arts today, students perform and believe as they are told to by their instructors? In addition, just as today, there were charlatans, frauds, and pseudo-masters around in the 1800’s. People who wanted to feel important, so they proclaimed themselves experts at a new way of doing something that did not need changing, and, and according to them, a way that was better than the traditional way of doing things.
Another reason the used vertical punches was because early boxing was a gentleman’s sport to be performed in a gentlemanly manner. If you did not box in the prescribed manner, you were ostracized, so all boxers fought in the prescribed manner. In addition to vertical punching, early boxers stood upright with their heads held high, they rarely blocked or used bobbing and weaving, they used very little footwork, they trained very little, they smoked a lot, and they partied and got drunk before fights. If the vertical punch as their great innovation, it was certainly the only one.