Why not more popular?

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) 

Colored reproduction ...

Open baffle colored? Surely you are confused. You might not like em, but colored they ain't...........

@mijostyn - I think an H-frame or W-frame design works a lot better for open baffle since it keeps the baffle size very small (barely bigger than the driver). My cabinets hardly vibrate at all (less than the sealed Velodyne subs I use in my home theater) and sound very natural to me. 

It is hard to make a speaker that is wanted and cherished by all audiophiles there are just far too many options for that to happen at this time of audio history.