About 20yrs ago.... you had a choice of 0, 1, 2, or 3yr contracts. Advertisements were confusing, but the option existed.
I think they went to the 0 and 3yr contract for simplification.
things changed when the iPhone came out..... the non-removable batteries were failing after 1 year. Also it became fashionable to change phones after one or two years. Many then wanted shorter contracts.