The added mass of the extra tube and key weight tends to harness the shrillness of the high register and some hear this as making the flute "darker" in sound. While the low B is increasingly used in repertoire, the extension of the tube has other effects as well. The longer tube creates more resistance in the low register, having a longer air column to "push." Some see this as positive; others prefer the ease of response on a C foot.
In the final analysis, we sell few C foot instruments as compared to B foots. In some models, the C foot is viewed as a special order.