As to whether it is "a deliberate policy or just some underlying cultural or historical bias?..." it may be that these issues are interwoven. I'm sure there is jingoism here, of course. I don't think we'll see other Counties than Cornwall and Dorsetshire out of Aoshima for these reasons but, at least, we wind up with two variants...which is a help. Whether they scale out properly is another issue, of course.
As I've mentioned earlier it simply can't be an issue of what sells, given that several Japanese manufacturers are modeling rather obtuse auxiliaries, ie. tankers, submarine tenders and so forth. That puts paid to the killjoys who argue that only what sells is made...and who, ironically--if not stupidly--buttress those manufacturers who choose not to cut molds on anything but a sure, if shortsighted, move. It would seem that the Japanese (and other Asian) manufacturers are looking at the long haul rather than the next quarter.
I agree with you: it would be interesting to see how the decisions are arrived at; incidentally, I'm simply ecstatic that the Swift was finally modeled (aside from the old Hawk kit).