It's a leftover from windowed UI design. In an OS level window requestor, the "affirmative" button is traditionally on the left, and the "negative" button is on

It's a leftover from windowed UI design. In an OS level window requestor, the "affirmative" button is traditionally on the left, and the "negative" button is on the right. Progressive UIs were not common, except in wizards. When forms were implemented, it was important to keep the behavior consistent with what people experienced in their OS. Now that we have touch phones, the situation has changed, but the best practices have not.