Hi Steve!
Thinking about the idea of having the brakes behind the fork...
Placing the brake there means under braking, the brake posts, caliper arms and pads all tend to draw toward the rim, rather than away, as when they're mounted ahead of the fork. The fork serves as its own brake booster without need to add one.
Looking at the braking on Sherpa compared to that on the Nomad, it seems to work out that way in practice.
My guess is this also works better if the fork has heavy gauge blades, as on the Nomad, which seems to be the only model where the brakes are still behind the fork. It is also Thorn's heaviest-duty, expedition touring bike. In that application, I've found it works great for me with extremely heavy loads.
Speculation again, but I think the range of lighter fork options on the Raven was also a factor. The lighter forks likely wouldn't work as well with rear brakes, and to have some setup with brakes on the front and others on the rear rear would prevent easy switching between them.
Best,
Dan. (...whose remote viewing does not always have clear reception)