Well, originally, it was an ESPN gig because they were the only ones broadcasting it. Think Pergament's story mentions that it's actually the Hall's decision, though, kind of like the control the Augusta C.C. powers-that-be keep over CBS for the Masters broadcasts.
Perhaps Berman thought that if he emceed, it would de-emphasize his presentation speech. Or maybe he feels that since he's part of the show (as Ralph's presenter), he shouldn't also be working for someone else that day. Serving two masters, and all that. If I get a chance to see him out there, I'll try to remember to ask him ... or, since I'll be outside the ropes, maybe I can get one of the guys with credentials to mention it.