It has nothing to do with the general audience being "dumb as a rock", it's just a common challenge that even real life zoos face. Multiple zoo design books mention the problems of exhibiting lesser known animals, some even mention how something as common as bird aviaries often don't retain the audiences attention unlike larger animals like elephants do. It's a simple fact, and it's not really weird that that translates to a zoo game as well.
You can even see it in the rosters of zoo games of the past decades. If you look at them and you compare them to Planet Zoo, there's so much overlap for a reason. The popular animals and less popular animals from a decade ago are still pretty much the same, with some exceptions here and there (which was bound the happen with the internet of course). It's not surprising really.
Planet Zoo seems to still be doing well in sales, but other than the cumulative sales we don't really have much information about how well packs are doing. Yes, there's the steam charts that give you some indication, but it's not a 1:1 ratio either. We'll never really know which packs sold really well and which didn't.
I wouldn't say there's nothing left, I wouldn't also go as far as say that every animal
@KönigDerKaffeebohnen has mentioned would really capture a large enough audience either. With what is possible in the game right now, I'd say you could have done one more full year of DLC, but not without the risk losing money as well.
If you'd introduce fish and birds however you could go for at least a year or two; but you don't want to start a new game with only the prospects of like two years of DLC after you had your previous game have 5 years of it.
100% this.
Larger expansion packs take more time to make and you have to really deliver to be able to pull that off (for instance No Man's Sky). People here are already pressed because we didn't get a new DLC in 3 months after the previous one, if you want bigger packs you're going to have to wait longer and I don't think a lot of people want that.
Looking at the wait time we had in ZT2 and the content we got with an expansion pack; Planet Zoo delivers a hell of a lot more in a year than ZT2 ever did. I'm perfectly fine with having it split over 3 months, it keeps the game much more interesting.