Why not more popular?

Lets see, since 1978 I have owned Acoustat Model X, Acoutstat Monitor 4, Acoustat 2+2, Magnepan Tympany III, Apogee Diva, back to Acoustat 2+2 and finally Sound Labs 645-8. I have worked at Sound Components and Luskin's both in Miami, FL and have listen closely to lord knows how many "regular" loudspeakers. 

@erik_squires , you are getting close. When you take the ESL concept and turn it into a line source it is like turbo charging a car. 

To do open baffle correctly you have to have a lot of loudspeaker and all drivers have to radiate equally from the front and back or you will not get appropriate cancelation at the sides. Forget about bass under 100 Hz. It will be so lumpy you won't even be able to correct it with room control. You might think this sounds OK. It does not. Subwoofers are mandatory. A line source has to go from floor all the way to the ceiling to function as a line source over it's entire frequency range. You have to fully dampen the wall behind the speakers or you will lose detail and imaging. Making a line source that meets all of these requirements  with dynamic drivers is difficult. Planar Magnetics could come close but will never achieve the performance of a well designed ESL. The distortion produced by an ESL is orders of magnitude lower than other drivers. Their diaphragm is so light that it almost matches the impedance of air. The entire diaphragm is controlled by the signal. The large surface area is capable of transferring huge amounts of acoustic energy to the air (as long as there is no bass below 100 Hz). Only horns can match this.
My guess is that my current system will have no trouble getting to 110 dB.
I'll have to wear hearing protectors to find out. At regular volumes thing like snare drum hit will slap you in the chest. 

So, what is the main difference? When I listen to music on dynamic speakers, baffled or not I know I am listening to a reproduction. When I am listening to a good live recording on big ESLs, I am there. The volume and power are the same. The speakers and the walls disappear. 
The only downside is you have to deal with the size which for many people is hard to swallow. 

There is no way to do an open baffle subwoofer right. Jaytor turn up the volume and put your hand on top of a subwoofer. It will be vibrating like crazy. That is called resonance or distortion. Yes, you will hear bass but it far from accurate. I am a dipole fanatic. I also started using subwoofers back in 1978 and currently build my own. I have made subwoofer similar to your using 4 12" drivers with a three inch thick baffle three feet wide. They made a lot of bass, totally inaccurate bass. They simply could not project very low frequencies. All they were at those frequencies were room shakers. I gave up on the design in three months. I couldn't even fix it with digital room control. You would have to make the baffles out of lead to keep them quiet. When you get a chance to listen to to a system with two or more JL Audio Fathom 113's you will get the picture. The best designs now use a balance force design. Kef does it with the blades. Magico with their big subs. They put a driver at opposite ends of a very stiff, heavy enclosure that operate in phase. This keeps the enclosure from vibrating and distortion levels are much lower.
Open baffle speakers for frequencies above 100 Hz can sound great if set up correctly with sound deadening on the wall behind the speaker. Room control helps a lot. As Erik suggests it would be an interesting project doing a set of open baffle line sources sort of like the old Pipe Dreams but open baffle and 7'10" tall. The hard part is the tweeters have to be closer than 1/2" or they will not function as a line source. A ribbon like the one found in Maggie's 20.7 would be ideal then a stack of 4" diamond drivers. With sealed subs and digital cross overs that could be incredible (but very expensive) 

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.