It's a decision from 2003 I think. It's mainly because I'm from the UK, so I'm extremely sensitive to the risk of people confusing DD-MM-YYYY and MM-DD-YYYY - the least ambiguous format is to use DD-Mon-YYYY, so I picked that for my URLs.
If I was designing my blog today I'd probably drop the day and month entirely, and go with /yyyy/unique-text-slug for the URLs.