The year my daughter was accepted to AU, the University of Delaware erroneously sent out e-mails to students informing them that they were accepted when in fact they were not. My D did not apply to Delaware, but decisions for Delaware that year came out the same weekend as AU's, and it was hard to avoid all the posts on CC by angry parents and students that weekend.
This year another school, I think in Minnesota, did the same thing. It is easy to make a mistake when sending out e-mails. AU may be resorting to snail mail to avoid making such a mistake. Or possibly they are trying to emulate what other schools are doing. Georgetown sends decisions by snail mail.