I would assume that the reason is that most counties do not have any caches (from some fairly fast investigations). But I can agree that this isn't a wishful effect from that. I will take a look to see if it can be improved.
Also, while at it, I reduced the quality of the Brazil county polygons a bit, so it should now work to view the whole countries counties at once.
EDIT: Or I realize, we can not fix this. We do not have any connections between regions and counties if no caches exists, not in the way the site is built. Fixing that would be several days of work. But I think since you can now view the whole country, it is not as big of an issue.