Because he already had some spread to his bones - RT is given as a curative treatment to men diagnosed early, or as a salvage treatment to mop up any left-behind bits after surgery.
The other time that RT is used (but in quite a different way - only one or two short blasts rather than 4 or 7 weeks) is to reduce pain from specific bone mets but fortunately, your dad doesn't seem to need that at this stage :-)