Why not more popular?

There are a number of open baffle speakers sold fully assembled.  I regularly see Emerald Physics for sale on Audiogon, and Nola speakers made a big splash in the audiophile world a few years ago and they're still around.  A google search will turn up more manufacturers, Spatial Audio and Kyron Audio, for example, so open baffle is not DIY only.

@tomcy6 - Clearly not just DIY. There are a few well regarded brands. But this is a tiny fraction of the number of box speaker brands, and most audio stores don't carry any. In fact, most audiophiles that I have talked to have never heard an open baffle speaker system (at least knowingly), which is why I posed the question.

FYI, in a former life, NOLA was the Alon speaker line of Acarian Systems.
NOLA is Alon spelled backward.
Both are the brainchildren of Carl Marchisotto, who has had an interesting audio career.   

My initial foray into a higher end audio system included Dahlquist DQ10, purchased after hearing at my first major audio show, thought the room with them best sound of show.
Later on became enamored with Alon house sound, owned a couple models over perhaps five years. In time I began to hear an incoherence I could never solve. Loved the mids and highs; fast, open, transparent. Bass another issue; round, slow, ponderous. Mids and highs on open baffle and bass in box didn't work imo.  I tried all manner of amplifer, could never get the coherence I sought. In the end I determined only biamping, tubes on top, ss on bass would solve speaker issue, but then we have amplifier coherence issue.
At that point I gave up on open baffle, did take a while to enjoy box speakers again. And now I've gone away from box again, big time into horns now.

I remember the DQ10 fondly, although I agree that the bass was their weakness. 

I haven't heard other full-range open baffle designs besides the ones I currently own. But I can say that the bass is definitely not a weakness in these speakers. I think the combination of drivers specifically designed for open baffle, plenty of radiating area, and servo control, really brings it home. 

But I will agree that this requires considerably more space than a conventional speaker. I have two large cabinets for each channel, and they absolutely can't be pushed up against the front wall. I'm fortunate to have a big enough dedicated room to let them shine.

From what I have heard from others, the Spatial speakers deliver excellent full-range performance in a more room (and WAF) friendly implementation.