Open Baffle. Why are they mostly limited to DIY?


I see a few hybrids from Vandersteen and Spatial Audio, but not much else. 
seanheis1
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
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!
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. 
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.