Why not horns?

The cabinet is a rear loaded horn I believe. It is used to extend the low end output of the DDD driver I believe.

A Walsh driver, including the ddd, transmits sound through wave bending for higher frequencies and operates pistonically for low frequencies, which is where the horn is applied much like it would be in any rear horn loaded design I believe.

"I believe" indicates that you are guessing. I don't know enough about the intricacies of speaker design to evaluate your speculation.
Maybe someone else can explain how it works. Anybody?

Mapman - My curiosity got the best of me so I checked out the GP website. The Unicorn does look fascinating. The efficiency is strangely low for a horn system and the power handling modest but I'm sure anything from that company has merit. Alas, I can't afford them.

My understanding is the Unicorn is GPs attempt to make a full range speaker using only their DDD Walsh driver. They accomplish it via horn loading the driver to provide the low end that would otherwise not be there in lieu of a separate driver.

I've never seen a Walsh driver that could be considered efficient and there is no horn loading of most of its frequency range possible (due to its omni nature) to help make it so . I suspect that accounts for low efficiency of these compared to many horns.

Macrojack,
The bass is horn-loaded. The upper frequencies are omnidirectional because of the driver construction. It is "low" efficiency. I find the GP Unikorns can sound sublime in the right room.

For more fun, take a look at the Duevel Bella Luna. It is a very interesting implementation. The top of the line is the Duevel Sirius, which both goes lower and is more efficient. But it ain't cheap.