vi has been around since the dawn of epoch time. It was one of the first editors used by UNIX hackers from days gone by. While it has been updated over the years to work on every flavor of *NIX out there, a look at its origins explain alot when you consider how unintuitive it is.
The first unix terminals didn't have 104 key keyboards. They were teletype machines that had paper coming out of the top instead of a monitor. Because of this limitation, a means had to be devised for it do all of the stuff that any modern editor needs to be able to do. Hence the Esc D commands for deleting.
vi is still around today because of its portablility. As I said, it runs on every *NIX out there. It has been upgraded over the years to add features and power, but its stays close to its roots in function to keep its portability intact. Unless you are a glutton for punishment, or have been weened on vi, I would defininately recommend the use of pico or emacs, over vi. While its power is undisputed, its ease of use leave much to be desired. All unix hackers should have a cursory understanding of vi as a just in case option when pico or emacs isn't loaded on the machine in question, but when given other options, a beginner should look elsewhere for his/her editing needs.
Oh and the answer you are looking for probably has something to do with keyboard mapping and the stty command in unix. Linux probably has the backspace key set to ctrl-h, whereas apple and darwin might not.