It's the company providing their customers what they asked for. Because of the way pilot licensing works, the airliners really want to field as few separate typ

It's the company providing their customers what they asked for. Because of the way pilot licensing works, the airliners really want to field as few separate types as they possibly can. If a Boeing executive goes to meet the people who buy his airplanes, upgrades that fit on existing type certificates is the first (and only) thing they ask for. The job of a CEO is sometimes to tell the customers no. It's just really hard to do.