For a good reason
There's not a good reason for dismissing someone singing at a lower register. It all boils down to whether the listener likes the song or not. Good singing is not always belting out vocals or hitting complex high notes. Christina Aguilera can belt out notes all day, but what separates her from a Whitney Houston is that Whitney had the control and restraint that most singers lack. Most singers with powerful voices get lost in that and forget the voice is also an instrument and it shouldn't just be runs all over the place that don't compliment or enhance the music.