Open Baffle. Why are they mostly limited to DIY?

Don't forget that the first open baffle speaker was the Dahlquist DQ-10. Woofer in a sealed box and 4 drivers (mid, upper-mid, treble, super-tweeter) mounted above in a horizontal array. A favorite of both Harry Pearson (TAS) and Peter Aczel (The Audio Critic). It resembled the Quad ESL (a favorite of John Dahlquist) and was about the same size. The DQ-10 was the first sophisticated speaker to appear in the mid-70's and was a commercial success! I still own an upgraded pair.

Open baffle designs remained niche market products because of the inherent conservatism of the speaker companies.  John Dahlquist later produced the DQ-20 and DQ-30. After he sold his company and it closed his associate Carl Marchisotta went on to found Alon (now Nola) and continues to develop and market OB speakers. 

Lest we forget (again!) Bob Carver developed and marketed (successfully!)   the Carver Amazing Speaker in the mid-80's. Two versions, the Platinum (4 12inch woofers and a 60inch ribbon) and the Silver (3 12inch woofers and a 48inch ribbon). Both designs were OB trapezoid shaped. And sounded better than anything else of that era! Still competitive today against multi-kilobuck speakers!

Well, they are different, they are large, and they need a lot of power. Other often praised examples would be the designs by Siegfried Linkwitz: http://www.linkwitzlab.com/index.html

GR Research offers many OB speakers, but they are offered as kits only. You get the drivers, x/o parts, baffle damping material (NoRez), and baffle making plans. You then either make the OB baffles or buy them from one of the woodworker/cabinet builders who offer them to GR Research customers. If you can solder, you can build the simple cross-overs; if not, there are GR Research Audiocircle Forum members who will do it for you. The open, transparent sound of OB speakers (no enclosure colorations and resonances) at very low cost. If you don't have room for the generally large planars (ESL, magnetic-planar, ribbons), the smaller-sized OB's are a great alternative.