To ensure the Unix-like standard of maximum backwards compatibility. Vi (the original, before Vim) was made in an age where computer keyboards might not have had luxury features like arrow keys, but did have alphanumeric keys and minimally competent users. It has worked for almost half a century, so unless you're Microsoft, why would you change it?

Even today, many people prefer it because you don't have to move your hand far away from the home row while typing or navigating, and the modality gives the user a much greater toolkit (seriously, I just about nutted when I discovered d i "). Not having to rely on modifiers and the arrow keys also reduces the risk of the repeated stress injury known as Emacs pinky.