Because Jennifer Hale does an outstanding job. I don't care if I get flak for this, but at this point, I'd take her over either David Hayter or Cam Clarke as the best VA in the bis. The VA who does male Shep doesn't speak with nearly the emotional range. Every line delivery I've heard from him sounds the same.
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Yoblazer: http://i43.tinypic.com/25z1non.jpg
Watch and you'll see... SuperNiceDog own me... in the Guru!