Why not more popular?

You know, conceptually I like open baffle speakers a lot more than I like ESLs.

Ages ago I was smitten with the detail and air electrostatic loudspeakers (ESL) s could provide, timbral accuracy be damned.

Open Baffle speakers, like line arrays, have the potential to overcome all those shortcomings while being overall easier to drive than ESLs, not to mention a wider sweet spot.

I have to agree with @wrm57 . I also purchased a pair of well known open baffle speakers. I describe them as the best, most boring speaker I ever owned. They certainly were very detailed and had a great sound stage but just didn’t have the same warm, engaging, musical feeling of a box speaker. I own Legacy’s now.

I think they sound really good at volumes at or below conversation levels.

And while there can be some details that aren't as clear as other manufacturers I think the sound stage is very good.

My friend has a set and there's a difference between his system and mine and they both encourage music enjoyment.

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