It wouldn't surprise me if the Termana Gazetteer didn't do so hot; part of the success of the Ghelspad one would be, in my opinion, the fact that before it was printed, there really weren't any details on the world outside of the few city books (I think Burok Torn might have been out before the Gazetteer, though don't quote me on that) and the sketchy sentences here and there in all the other books. When it came out, it was the setting book if you wanted to play in the Scarred Lands. Termana didn't have that going for it; it also has that "other continent" thing going for it that keeps the Forgotten Realms popular, and settings like Al-Qadim, Maztica, and so on, fairly obscure and now unpublished. Those who felt obligated to buy Ghelspad, wouldn't feel the same about Termana.
Maybe that's not the case, but, it makes sense.
Personally, so long as the Asherak hardcover comes with a map, I'll be happy. My wallet would certainly appreciate not buying a Gazetteer AND a hardcover book about the same topic; I've both the other Gazetteers, but do believe it to be a good thing if they just skip'em and put all the info in one book.