5 not V, for mental disorders

The previous versions of the American Psychiatric Association (APA)’s Diagnostic and Statistical Manual of Mental Disorders were called DSM-I, DSM-II, DSM-III, and DSM-IV – but the new one is being named DSM-5.
It seems the APA have, for reasons not yet clear, decided that Roman numerals are out for the 2013 edition. Can any readers offer an explanation?