It's funny, but I always assumed the SEA pack was a big hit because it had 8 animals. There were certainly a lot of people expressing approval of this in the forums, and it appears that it was enough of a success that the developers did another 8-animal pack for the most recent DLC.
I don't think any pack is going to bat 100% with the animals for most users. The thing is, aside from certain iconic species, people won't all agree about which ones were pleasant surprises versus disappointments. Someone said they didn't like dholes, but I was excited to see them. They're endangered canines that are present in some real life zoos. I'm not so thrilled about the model (they look too foxy to me), but then I love canines. I didn't expect the binturong, but it's cool to see them. I like the way the game sort of balances the go-to zoo animals one sees at most zoos with lesser known species that are more likely to be present at zoos that specialize in breeding rare and endangered animals.
I'd be interested in seeing the relative popularity of the DLCs to see if a pattern emerges, aside from a gradual drop off maybe (or is it possible that enough new players will discover the game to offset player attrition).
Oh for sure, animal packs can definitely be a way to go, but I do have to point out that most of the animals in the NA pack were fairly highly requested (5 of the 7 habitat animals), and Frontier leaned heavily into the more obvious and less obscure choices, even for the "surprises" like the Arctic Fox and Prairie Dog.
Meanwhile the SEA pack, while still a good pack, had a leaked animal that most of us assumed was coming, so it wasn't a surprise but I'm not sure how highly requested it was, 3 or possibly 4 fairly requested animals (though none that were as prevelant in requests as the five NA animals) , and the other 2 habitat animals (the Proboscis Monkey, and Dhole) that were surprise picks not many of us saw coming.
In short, the NA pack was likely a good selection to see how an animal pack of very popular animals could sell, to really get an idea of how these types of packs could sell; as opposed to the SEA pack that had comparatively not as highly requested animals. If Frontier had given us a Cougar (bc there was no way they weren't making it in), Prairie Dog, Arctic Fox, Black Bear, Coyote, North American River Otter and Big Horn Sheep, to pull up mostly moderately to not very requested animals, the pack probably wouldn't have gone over as well. To use a sports analogy, they hit a home run with the NA pack - it's probably the most "fan favorite" geared pack they've done yet.