Alright, then. I suppose I can't be really picky about the traders being this or that, so I'll lax a bit and let untagged, generic traders in as well. That and traders linked with Rheinland/Kusari companies.
EDIT: The explanation is that the companies involved Daumann and (Kishiro or Samura) can't really come around with the logistics needed. Well, in fact they can't, as far as we PCs are concerned.:P
EDIT2: This hurried attempt for outside help can be the cause for the information leak (pirates getting to know about this convoy).