I was editing my answer to a very similar question here: Why there is difference in pronunciation of words PUT and CUT? In fact, I think these questions should be merged. But I also think that the merged question should be re-opened, in case someone in the future has more useful information about this topic. Please post an answer explaining what you think.
The question is basically about the historical development of one part of the English language. It has a real answer supported by facts and not opinion; read Jon Hanna's post to see what it is.