Because it's more efficient?

Yep! It is more efficient but just in this very small frequency-band (eg. 50-60 Hz). Every Frequency higher or lower than that you don't really hear any more.
I got something like a "hole" in my sound. Some basses came very well but as soon as they ran out of the band-pass freq.band I could hardly hear them.
A closed box or bassreflex is more dynamic.

I don't want to say that are the only useful boxes. I just say what I saw in my case in praxis.

TeeMcBee
Got my Mk2! # 080000143