Why not more popular?

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!

Few things audiophiles buy are popular.

And even things that seem popular and acceptable mostly wouldn’t be so if the PIRC saw the invoice!

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)