Not surprised. If they actually accurately modelled the different shaped faces of the two different peoples it'd be twice as much work. Ethiopia would also have to be one of the least played countries... especially concidering it's difficulty.
The question also has to be asked, even if this dlc got a decent number of sales... how many people actually bought it for it's sake and how many bought it to be a completionist. So far with at least one dlc I fall into the second category and it'll probably be the same way with this one (as soon as I'm able to make purchases online again in a couple of days).
Ethiopia and Nubia were popular enough that Paradox added Coptic Pope and Miaphysite religion into the game after people had constantly complained that religious situation in Ethiopia is incorrect.
And it's not just limited to CKII. East Africa has sort of cult following among the Paradox players. In original CKI Nubia was popular choice (Ethiopia wasn't on the map) among the players who wanted the ultimate challenge. And in EUIII many people have conquered entire Africa and more as Ethiopia. I assume that Ethiopia is still considered as a good challenge also among the Vicky and HoI players too (don't have the latest version of these games, so I don't know for certain).