Because commenting is generally a different skillset than announcing, and what tends to happen when color analysts get shifted to play-by-play is that their first impulse is to analyze, which is not the announcer's job. I wouldn't feel comfortable trusting either Doug Christie or Kayte Hunter as a full-time play-by-play announcer, because I feel like they would get too wrapped up in wanting to break down the play on the court, instead of simply describing what is happening, in real time.