This isn’t very satisfying, but I would suspect simple language drift to explain it. Someone came up with one-on-one or one-to-one to describe this particular t

This isn’t very satisfying, but I would suspect simple language drift to explain it. Someone came up with one-on-one or one-to-one to describe this particular type of meeting and popularized its use. Eventually someone decided 1:1 was a good shorthand.