Loudspeaker Efficiency
If we compare two speakers of identical diameter and voice coil area, mounted in the same enclosure to eliminate differences in efficiency due to loading, horns, etc... besides resistive losses, most of the power goes into accelerating the loudspeaker cone back and forth, braking it, then accelerating it in the other direction, etc.

This means efficiency strongly depends on moving mass (voice coil, former, cone, dustcap, etc). 

So, high efficiency requires a very light moving mass, but then it is more difficult to make the cone stiff and properly damped.

There is always a compromise between stiffness, damping, moving mass, and cost.

If it is too light yet stiff, it'll lack damping, so it'll ring, exhibit bad sounding breakup modes, and maybe fail if someone cranks the volume to 11 and the speaker bottoms out. More damping reduces efficiency, and it also increases mass because it requires more material. If you still want it light, stiff, well damped, good sounding and efficient, then you'll need more expensive materials.

Also (and this is important)... say you make a very efficient speaker, now you have to put it in an enclosure. Then if you do the math... 

It is quite easy to have good efficiency at frequencies above, say, 2-300Hz. However, efficiency at lower frequencies requires much more enclosure volume. Basically you want all the springs (loudspeaker surround, spider, and air in the enclosure) to not be stiff so less power has to be expended to push the speaker cone against these spring forces. And if you want the air in a box to have lots of compliance, then you need a very big volume of it. This is why audiophiles with flea powered tube amps that still get great bass often have washing-machine sized speakers.



...and if you don't want a huge cabinet, then either you'll have to sacrifice bass extension and cut at 70Hz, or bass efficiency will be lower. Now, if you use a passive crossover and want the woofer and mid/tweeter to be well balanced, then you will have to pick drivers with the same efficiency as your woofer.... and the usual problem is, unless you build a washing machine sized box, the midrange/tweeter is usually too efficient so it has to be padded with a resistor to match the woofer! (or use active crossover)

So you see, it's a bit complicated.