How come Horn + woofer designs are not more popular?

@mrdecibel --

But a question for gw _smith. What are you referring to when you say " large mid horn " ? and " that’s an 8 inch bass horn wrapped up in a very small space " ? I am a 50 year Klipsch Heritage veteran ( not an employee ), and I am not understanding either of these statements. Would you please, care to explain ?

Maybe you’ve received the answers to your questions in the meantime, but poster @gw_smith wrote the following:

If I may add, that Paul Klipsch’s Klipschorn was all about making a woofer that COULD keep up with that large mid horn! (thats an 8’ bass horn wrapped up in a very small space!!)

To begin with he refers to, I believe, a bass system that can dynamically, transiently, sensitivity- and otherwise sonically compliment the midrange horn of the Khorn in a suitable fashion for an overall more coherent presentation, something that could have more easily gone haywire with a direct radiating solution, certainly in regards to maintaining the traits mentioned just above.

He’s then referring to the 8 feet (not inches) long bass horn path of the Khorn that’s folded in such a way to make for a fairly compact design, relatively speaking; as 1/4 wavelength horns we’re still faced with significant sizes if we’re to go anywhere near 30Hz, let alone deeper.

I think JBL M2 have too low sensitivity 92dB  for horn + 15" woofer speakers.
This fact kills they advantages over box speakers.
Yes, they should have better high and low frequencies extension compared to typical horn speakers, 
BUT
They will never sound as dynamic and alive as 97-100dB horn speakers and you can't use them with SET amplifiers.
I listen mostly jazz and classical music, and for this kind of music low mass old 15" woofers like where used in: old Altec, JBL L200, Tannoy Silver, Red suit better. 
New heavy 15" woofers more suit for rock and pop music but they don't breathing and sound more simplified on midrange. 

Speaking purely subjectively, for me speaker brands highlighted different things.  As a musician, I prefer to evaluate audio with music that doesn't rely on amplification for its realization, hence acoustic instruments.  Since I was a young teen, I've loved the way piano sounds on JBLs (I've had a Steinway grand almost since then).  I was blown away when I heard Wagner on a pair of Bozak Concert Grands.  Strings always had an unmatched warmth for me on KLH.

I've owned and loved JBLs since a teenager (over 50 years ago!), acquiring a bass reflex system with D130s alone at first, adding "potato masher" horns around 1970.  I still have them, and recently hooked in a pair of O75s.  In the early 1990s, I lucked upon a Paragon and vintage McIntosh MC240 & MC275 amps.  At first, I thought the sound impressive, but a bit harsh with weak bass.  The harshness increased as one moved closer to the horns, of course.  Subsequently, I learned that the previous owner listened at less than a moderate level (he had a cheap L Pad attached to it for convenient volume adjustment!)  I added a graphic equalizer and used it for years until I noticed that the (stiff?) 150C woofers had blossomed.  I took the equalizer offline and added a pair of JBL 2405s in parallel with the O75s.

My friends, mostly professional musicians, never failed to be impressed.  However, for me, there was yet a ways to go.  Five years ago I installed a McIntosh MEN220, "Room Perfect" component—bi-amped with an MC2105 for the low end and the MC275 for the horns and tweeters.  After running the room correction algorithm, I found that a great majority of what I had perceived as shortcomings were essentially gone.  There's nothing like live music, but this system continues to give me a thrill.

Horns can be the last loudspeaker you would ever need to buy good ones are not obsoleted as quickly or at all like nearly all conventional dynamics and estats.  They hold up well with time unlike the many others and can be easily repaired or upgraded. Horn based systems are great at producing a near real sound that the other types struggle to produce. But they are large take skill and knowledge to set up and are a bit harder to source. The best in horns is fully horn loading and most consider this just a bit to much to pull off. So many of those who claim to have heard a horn system most likely only heard hybrids not full horn loading. 

My Heresy IIIs were extremely easy to set up...you take them out of the box, put them in front of you, attach the wires...