I imagine backwards compat is a big part, but also:

Dir=auto means ignore what came previously when trying to guess what direction this text is. Not setting a dir attribute means the browser should use the text in the parent/previous sibling element for figuring out what direction this element's text is. Auto might make sense as a default for a <textarea> where user input is disconnected from the html, but would be a horrible default for most tags like <b>