Why not more popular?

My initial foray into a higher end audio system included Dahlquist DQ10, purchased after hearing at my first major audio show, thought the room with them best sound of show.
Later on became enamored with Alon house sound, owned a couple models over perhaps five years. In time I began to hear an incoherence I could never solve. Loved the mids and highs; fast, open, transparent. Bass another issue; round, slow, ponderous. Mids and highs on open baffle and bass in box didn't work imo.  I tried all manner of amplifer, could never get the coherence I sought. In the end I determined only biamping, tubes on top, ss on bass would solve speaker issue, but then we have amplifier coherence issue.
At that point I gave up on open baffle, did take a while to enjoy box speakers again. And now I've gone away from box again, big time into horns now.

I remember the DQ10 fondly, although I agree that the bass was their weakness. 

I haven't heard other full-range open baffle designs besides the ones I currently own. But I can say that the bass is definitely not a weakness in these speakers. I think the combination of drivers specifically designed for open baffle, plenty of radiating area, and servo control, really brings it home. 

But I will agree that this requires considerably more space than a conventional speaker. I have two large cabinets for each channel, and they absolutely can't be pushed up against the front wall. I'm fortunate to have a big enough dedicated room to let them shine.

From what I have heard from others, the Spatial speakers deliver excellent full-range performance in a more room (and WAF) friendly implementation. 

Maybe because of room limitations as they need to be placed out into the room.

Here is my Q have you heard a FR, single source driver? 
No, WEll you should. 

bdp24, maybe I am just an old guy and you do seem very informed but in my day open baffle meant just that and was not driver type limited. ESLs and planar magnetics are open baffle speakers and yes they are dipoles but so are any dynamic driver open to the rear. 

Next, if you think you can adjust the bass response of an open baffle woofer by adjusting the size of the baffle? Lets see, the wavelength of 100 Hz is about 10 feet. Any open baffle speaker of reasonable size is going to have bass cancelation issues regardless of the baffle size. That is just lay intuition. A driver with more surface area will produce more bass not a bigger baffle. So you see what Magnepan does with it's woofer panels. It makes the woofer diaphragms larger, many times the surface area of a 15" dynamic driver. But even with large diaphragms there are serious problems that prevent the production of adequately accurate and powerful bass in a room. Sound Labs has experimented with ESL subwoofers 7 X 7 feet, 49 square feet. Roger West is now working with old 30 inch EV drivers. Now he is a madman. 

Sand infill speakers have been around since the 50's. Warfdale I think was the first. If the final thickness of the enclosure wall is say 2" making a 2" thick MDF enclosure works just as well if not better. The primary resonance of a subwoofer is not the enclosure walls flexing, it is the entire subwoofer shaking. The way to stop this is to simply use opposing drivers in a "balanced force" configuration. The forces cancel out. Kef does this with the Blade, Magico with their big subwoofers. Boxes are easy to make but they make lousy subwoofer enclosures, better than an open baffle but that is about it. The ultimate subwoofer enclosure would be a sphere. Very difficult to build and apply. Next however is a cylinder. Wall stiffness second only to a sphere and easy to apply. Just stick a driver in each end and you have an opposing "balanced force" design in an enclosure that can be way stiffer than any box. No sand required.
I am a big fan of open baffle speakers using dynamic drivers in a 2 way configuration crossing down to an enclosed subwoofer. The open crossover can be made to look like art work. I recent heard a wonderful pair using Accuton drivers in a 1.5" thick quartz baffle board. Very cool looking. Instead of a stand they were hung on chains. What a great idea.