Why not more popular?

Open baffle speakers are an entry drug to dipoles, which leads one to omnis......*smirk*G*

One has to 'listen outside the box', Literally....

(btw, I have and still Do own some 'boxes'....one needs to remind oneself what they left behind.....and Why...)

Have dipoles, PAP’s, most likely, never going back to boxes. Like a good woman, when it hits your soul, one only listens to the music. 

Spatial X5. Wonderful!

My only experience was owning a pair of large Alon speakers where the mid and high range drivers were mounted on an open baffle.  I really tried to like those speakers (over several years and upgrades) but never got to a sound that seemed right to me (maybe a perceived lack of drive and dynamics).  I basically ended up in the same place as @tubebuffer - 

"Open baffle remind me of fat gym teacher. Something just not right."

   

I owned a pair of EP KCII Pro's for the better part of a year.  I knew I was not able to set them up ideally--could only place them 30" off the front wall.  Even so, they imaged wonderfully--of course, depth was limited but excellent height and width.  They also had explosive dynamics.

OB bass is very inefficient bass, due to wave cancellation.  So you need really big drivers placed well out into the room to mitigate this.  So in my experience, that tight, fast OB bass could also be described as a major lack of bass.  So that required subs.  

The deal breaker for me was a peak around 3-4khz that would rear its head on too many recordings.  So out they went, replaced by a wholly satisfying box loudspeaker.

Like many things in life, it's not the pathway you choose, but how you execute that choice.