Why not more popular?

Oh, and by the way:

The Magnepan "Concept" loudspeaker---the 30.7 For Condos---is basically not that different from Danny’s NEO Line Source/OB Sub design.

The Magnepan Concept has a smaller version of the 30.7’s midrange/tweeter panel, with an OB/Dipole woofer assembly in place of the 30.7’s large woofer/bass panel. That OB woofer has multiple small-diameter woofers (6.5", I believe)---six per side, iirc. And dipole-cancellation compensation, not unlike the GR Research/Rythmik.

Reportedly to sell for less than the 30.7, if it is ever put into production. For now, a pair of the GRR/Rythmik OB Subs, with a planar of your choice (ESL, magnetic-planar, ribbon), or an OB loudspeaker (one of the GR Research kits if you’re adventurous and confident, a Spatial if you’re not) will get you very close.

If size matters, the ET LFT-8b is only 13" wide and 5’ tall, much smaller than the Magnepan 3.7i. And it’s LFT magnetic-planar drivers are push-pull designs, with magnets on both sides of the Mylar diaphram for ESL-level distortion, unlike the single-ended 3.7i midrange driver. But the 3.7i does have a nice ribbon tweeter. The MG1.7i, only $300 cheaper than the LFT-8b, suffers pretty badly in comparison to the LFT-8b. I did that comparison.

Some of the comments are regarding open baffle seem, honestly, uninformed. To compare yesterday’s OB without listening to today’s OB and saying it’s a fad is pretty funny or sad. Box speaker’s have several limitations by design which OB resolves quite nicely. Yes, they look different, but if you have an open mind, truly listen, you will be well rewarded.  I did and created this review https://youtu.be/R9VeZOdatao

@bdp24 - thanks for adding all the detail. Danny's OB subwoofers are also used by GT Audio Works to great effect. These systems have received considerable praise at various audio shows over the past few years. 

The company that Danny worked most closely with that used the  Bohlender Graebener drivers was Serenity Acoustics. The company that acquired BG significantly reduced availability of the drivers (they used them in their own theatrical and pro sound reinforcement systems), which forced Serenity out of business. 

The specific models that I'm familiar with used the BG NEO10 and NEO3. Danny has been able to develop his own NEO3 replacement, which he uses in a number of his products, including the NX-Oticas which I currently own. He is also trying to create a replacement for the NEO10, but so far he has not been satisfied with the results of his prototypes.

Parts Express is now sourcing a similar driver to the NEO10 (and NEO8), but this driver does not measure as well as the BG NEO10, and also doesn't play as low in frequency, so can't be used as a substitute in Danny's Serenity designs. 

The Serenity Super 7 (which used a pair of 12" servo subs, four NEO10s and one NEO3) sold for $20,000 during it's short market life (before Serenity went under) and was (is) well regarded.

The Serenity Line Force, which used 6 NEO10s and 16 NEO3s (for each channel) in a line source configuration, along with a separate subwoofer cabinet with configurations of 2 to 4 12" servo drivers, was only shown in prototype form at shows, and used a machined aluminum open-baffle cabinet. It was expected to sell for $40K to $50K. 

Danny has continued development on both these models (when the BG NEO 10 was still available through Parts Express) to adapt them to use his NEO3 driver. The Line Source cabinets were re-engineered to use a composite material instead of aluminum which has reduced the machining cost considerably (although they are still quite expensive to produce).

I have been lucky enough to acquire enough BG Neo10 drivers from Parts Express before they became unavailable to build a set of Line Forces. I hope to have them completed later this year. 

I had a pair of Alons that had the tweeter and midrange mount on top of the woofer, which was inside of a box. You couldn't tell because it had an interesting angled grill cover so you couldn't see there was no box holding those drivers. They had an amazingly big sweet spot and overall they were very easy to listen to. Kept them for 25 years. 

Just last year replaced them with a pair of KEF R500s that are more detailed, dynamic and have WAY more bass, tighter, deeper. Not quite as big a sweet spot, but still very good in that regard.

I don't think you should generalize about the speaker type because each one has a different design. By and large though, total OBs can benefit strongly form subwoofers, which makes the whole system into a bigger more complicated deal.