Interesting question. There may be a contractual reason, or maybe they want to give Barnett some time on TV.
I never thought I'd say this, but Dave Barnett is a better play-by-play announcer than Rhadigan. But that's mainly because Rhadigan has no previous experience. It's all OJT for him. I think this is one area Nolan really screwed up - letting Josh Lewin go was a big mistake.