Why low sensitivity speakers?

Karls is right on the money. The greater the cabinet size the easier it is to move a lot of air, required for bass. It also enables you to use more than one or a larger bass driver, both of which add to the efficiency of moving a lot of air. You can make a comprimise between bass extension and frequency. If you choose a higher roll off frequency, the drivers do not have to use their efficiency to move the massive amount of air required for the lowest frequencies. I'm not a speaker designer but I think it's approximately a squared function, so rolling off can make a huge difference in efficiency.

Perhaps my explaination was not clear. It is not the driver response, but rather the crossover that makes the efficiency (thus sensitivity) change. Gs556 analogy is really excelent and as to how it applies to speakers, it's back to what I said in the beginning. Lower frequencies require more movement of air to achieve the same SPL, thus they are the "heavier bottles". Great analogy Gs5556.