Why low sensitivity speakers?

You can pick any two of these three options:
1. good bass
2. small cabinet
3. high sensitivity
This is an immutable law of physics. Since many people want the first two due to decorating and other domestic considerations, the third is the one that gets left behind. And since watts are relatively cheap, it is not a problem. High-sensitivity speakers have their own virtues, but nothing will ever change the above law.

What he said, and probably also because not enough attention has been paid to the potential of modified horn-coupling strategies in audiophile speakers (except in the SET fringe), as wide dispersion and flat power response have seemingly gotten priority over wide dynamic range and low distortion, for reasons unknown to me.

Karls,
Can you (or anyone else) explain why good bass, cabinet size and sensitivity are tradeoffs? Thanks.

what do people consider high Efficiency?

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.