It happened because the American style was developed for use of s spoon and knife? Before the widespread adoption of the fork.
The spoon was used in the nondominant hand to steady the good while it was cut with the knife in the dominant hand. Then the knife was out down and the spoon was switched to the dominant hand for eating.
When the fork was adopted, it just took the place of the spoon.
The European style was developed after widespread adoption of the fork and was thus created Dow officially for the knife and fork.
Speaking as an American, the American style is idiotic and as a child I refused to employ it. I came up with my own method which I learned much later was the European method.
Just to add, I believe the prior practice in both Europe and America was to use the knife in the dominant hand and the fingers of the nondominant hand.
Adoption of the fork in Europe was condemned by the church and other authorities as decadent and unnatural. (God gave you fingers, and such)
It was popularized by Europeans who wore large collar ruffs and found using the fork an easier way to keep their clothes clean.
(Actually, I don’t know if my personal style is exactly European. The fork stays in my dominant hand and I use my nondominant hand for the knife. Fork is preferably tines down, but can be used tines up for scooping.)
I was defiantly sat down and taught how to use a knife and fork on multiple occasions. I learned both the European and American style although I prefer the American style. It seems to me somewhat low class to do otherwise.