My guess is that one of 2 situations happened:
1. A lot of people drilled holes in a coin and made jewelry from them. Some did it because they liked the look, where poorer people (like my ancestors) did it so they knew they always had some emergency cash on hand.
2. And this is the one I believe...some guy needed it for his set and instead of paying the house payment bought the coin for a great price. The house payment bounced, the wife found out, divorced him and drilled a hole in it for her own personal necklace. I believe this because my wife would do the same thing to me if the coin collecting was before the bills.