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. 

Non-box speakers often have qualities that I didn’t think one could find in box speakers eg a certain openness, etc.
Non-box speakers:

—open baffle dynamics (earliest Dahlquest DQ 10), spatial audio, pure audio project, tri-art etc.
—electrostatics eg Martin Logan.
—Maggie’s
—omnis eg ohm, $25k space pods, etc
—Bipoles (old mirages). 

Clayton Saw (spatial) says his business is booming. If you like electrostatics eg Martin Logan, you like them, but they have been around forever and have nothing to do with the current popularity of open baffle dynamics eg spatial audio.
Interestingly, I bought a pair of bookshelf speakers (LSA) which have wide dispersion and have some of the same qualities

I have open baffle subwoofers that sound fantastic in my room. They seem to have way fewer problems exciting room nodes than conventional sealed or ported box designs. Probably similar to what can be accomplished with a swarm, but without "pressurizing" the room. The bass sounds more natural than I've heard from box (sub)woofers - I think because it is creating bass more similar to the way an instrument does. 

Using Room EQ Wizard, There are a couple of very narrow nulls at the listening position that are about 8db down due to room nodes, but 1/12 octave smoothing gives me response that is within a couple db from 18Hz up. 

My sub towers aren't small (three 12" servo controlled drivers per side), but the bass performance is worth it to me. 

https://systems.audiogon.com/systems/8726

Few things audiophiles buy are popular. 

Hello jaytor! Open baffle speakers are harder to place properly in a room than "boxed" speakers. My big system features the Linkwitz LX521-4 speakers: open baffle, 4 way, one amp for each "way" (eleven channels total - a 5.0 setup) and it's wonderful. Try putting a rug on the wall (hanging) behind the speakers if you're having any trouble. Thick, soft and fuzzy is best. I have two other systems using open baffle (Magnepan LRS and DIY) speakers. I love them! BUT - when a person is looking at speakers, they see the raw drivers, no nice cabinets, and they see the price tags. They know (or can easily look up) the price of the raw speakers themselves and wonder "Why am I paying XX thousand dollars for drivers that cost only 20% (or less) of that?" They forget the cost of the crossovers (lots of copper, not cheap), the packaging (much harder) & shipping, and the cost of the labor to put the thing together. It may not seem like a good value. Professor Linkwitz called his best system "521" because he tried many many baffle shapes and didn't get the "right" one until the 21st of May! "Bud" Fried's (the father of the transmission line) wife complained that she could never invite people over because the living room was always full of wood boxes and sawdust from her husband's experiments. Getting anything right takes time and money and effort.

Done right, open baffle speakers (remember the Dahlquist D-10 and some early sand-filled flat panel Wharfdales?) sound wonderful just because there is no box, no enclosure colorations. Many audiophiles are men with wives; and I don't need to finish this paragraph, do I? They just don't look like furniture (the speakers, not the wives). "You're not putting that thing in MY living room!" can be heard in many homes! I am blessed with a very wondrful and tolerant bride! Keep listening!