As Poser said, it's a combination of cost and reliability.
When I bought my first unit back in 86 or 87, they were about 600 new, RETAIL. The 27% OD peaked my interest. What I discovered was that it dropped the RPMs TOO MUCH for my liking, below the best engine RPMS for my rig, resulting in minimal mileage improvements.
The unit failed after approximately 8 months. As I recall, there were 3 different failures, one of which was prolly my fault, the other two, anybody's guess.
I've disassembled and rebuilt about half a dozen units since I've had my shop, and had more of an opportunity to evaluate them against other gearboxes. There are definitely things that can be done during the building of the units to improve their chances of survival. There are still weaknesses.
One is the mainshafts, which are hollow, and nested one inside the other. Especially on the 4 speed models, the internal splines on the imput shaft wear out quickly, as their cross section is not nearly wide enough to distribute the load from the tranny. Also, because of space considerations, the roller bearings that support the output shaft on the imput shaft are extremely small in diameter. I have seen a number of these bearings fail, with resulting galling of the imput and output shafts as well.
The three shift dogs that help the shift sleeve 'float' its way across from standard drive to overdrive are hollow and prone to breaking off part of one end. The snap rings that hold the shift dogs against the sleeve are turned out at their ends, with the turned out sections riding in the hollows of the dogs. Consequently, when the ends break off of the dogs, the snap rings come out of place, the dogs cant to one side, and the OD will no longer shift. Purely luck of the draw whether or not you will be stuck IN or OUT of overdrive.
The most common failure is the internal locknut which secures the imput shaft of the OD to the tailshaft of the tranny. It is EXTERNALLY threaded to the INSIDE of the imput shaft, and is to be assembled with loctite. I have heard of several locknuts that came loose anyways!
I have NEVER offered a warranty for servicing a Fairey OD, but the last two customers that had me build them are both very satisfied.
Hth
Mark A.