I have a Linhof 2x3 Vb, the last production model of this wonderful camera with 55, 100, and 180mm cammed lenses (Grandagon and Sironar S).
It is basically a 2/3 scaled down version of the 4x5 Master Technika with a rotating roll film back (6x9 in my case) with the option of both rangefinder and ground-glass focusing. Composing and focusing are in different windows (as in the MT), but one quickly gets used to this when shooting hand-held.
My main camera is a 4x5 Linhof Master Technika, but I never travel with it. The 2x3 is more portable and compact, and requires a smaller and lighter tripod (and bag!). I am very pleased with the results from 6x9 negatives. The quality of 16x20" prints come relatively close to 4x5 quality.
As to why this 2x3 camera does not appear to be popular, it is a simple matter that it was always very expensive new, and priced the same as a 4x5 MT. It was never a mass-produced camera, but a specialized niche product, and priced accordingly.
The 2x3 Linhof Technika is a wonderfully designed and engineered camera from the pinnacle of the glory days of high-end analog photography.