I'm probably wrong, but my first suspicion was that they're doing this specifically to create a market for the tektro 559. Both that brake and the paul racer WERE developed specifically for Rivendell, so perhaps they feel obligated. Grant has often said "buy something if you appreciate it, or it might not be around when you want it " or something to that effect. I get that.
I have one of each (tektro and paul). I don't have as strong negative feelings against them as others do, but I always thought the chief complaint was poor leverage and stopping power - more than lack of quality or ease of wheel removal.
For me, it's kind of the whole package: Cantis are stronger, better looking (subjective), accommodate fenders better, are friendlier for fat or knobby tires, and are easier to fine tune adjustment (maybe subjective again). I know that not everybody has the same needs as me though.
The other thought I had was that the somewhat awkward cable routing - necessitated by the curved/dropped top tube - might have had something to do with the decision to ditch cantis in the case of the charlie. But, as others have pointed out, there are tidy ways to solve that. And linear pull brakes.