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!