The real answer is because of the dominant hand.
What we know as wristwatches first began to become popular around the time of the Boer War. Back then, they were literally nothing more than a custom fitted leather pouch for a common pocket watch that could strap to your arm. Officers used watches to help coordinate military activities, and they were fairly large and awkward. These pocket watches were big because they had huge balance wheels that will break very easily if you bang the watch against something. Watches at that time weren't hugely expensive, they were still in the same class as a buying a modern house appliance. So, they were worn on the non-dominant hand trying to protect the watch from a bump or scratch.
Moreover, you usually use a lot of forces on your preferred hand for your activities, your wrist’s muscle will exercise, if you wear watches, bracelet, etc on preferred hand. There are a few consequences: (i) Excess exercise will break your watch’s bracelet; (ii) If the bracelet is not broken, your wrist will get hurt; (iii) your watch might get bumped a lot, thus damaging or scratching it. Which is why people prefer to wear them on the left hand, less dominant one. Or the other way around for left handed people, they will wear them on their right hand, since it is their less dominant hand.
By Robert Wrigley
(Vintage watch collector and amateur watchmaker)