The reason men historically have worn their watches on their left wrist is due to a simple matter of practicality. Most are right-handed and therefore have an easier time seeing the time when the watch is located on their left wrist.
Watches are traditionally worn on the wrist of the non-dominant hand so it is not necessarily on the left wrist. There are simply more right-handed people than left-handed. Also, this general rule of thumb is not exclusive to men, but both men and women tend to follow this pattern for the ease of seeing the time, as well as protection of the watch, since most work is done with the dominant hand.