That was the case indeed, at least at launch. However, they later included relatively more active animals like Diamondback Terrapins with looped animations, which is definitely a move in the direction of including more active exhibit animals. For instance I wasn't expecting active Turtles to be added to the exhibit system but they eventually did. Meerkats and Prairie Dogs were also seen as too small for habitats by many forum members, but they were eventually included in the game as habitat animals. My point here is, the developers seem to like to stretch the limits of both enclosure types set at launch, where possible.
All in all they still use the same rules as they did at launch.
Exhibit boxes are more about conveying animals realistically with a limited amount of animations rather than about conveying inactive animals. The benefit of looped animations is that they don't require a complex behavior system at all, but are simply limited to being essentially animated scenery, just one that has some more interactivity than other scenery. It's important to keep in mind that
all animals in the exhibit boxes have looped animations. Some just move around more, other just sit and breath. But all of them are that and nothing more. From a technical standpoint, there's no major difference between the terrapin and other exhibit animals.
The fact that small "inactive" animals like small reptiles, amphibians and insects fall in the exhibit box category is therefor not surprising. They are animals that in zoos don't tend to move around much, only occasionally you can see them move but 80% of the time you as a guest see them they remain still. So that's why they fit perfectly within the looped animation box system that exhibits are, with a limited amount of animations you can still get a realistic representation of the animal.
And that's why larger reptiles, which in zoos tend to move more often than their smaller counterparts, are not put in exhibit boxes but are in larger habitats as well. And the same logic again applies to smaller mammals. The whole benefit of the box system falls flat when you need to add so many looped animations to realistically convey a small really active animal, so I can see why Frontier doesn't do that and why they stick with that decision. Because at the end of the day, developers also have their own view on the direction they want to see the game go in.
On top of that, Frontier seems to prefer the customization factor that habitats bring. We can see it in many, many many of their decisions. They could have gone for prefab shelters only, but gave us the option to make all of those ourselves. They could have gone for prefab climbing structures, but allowed us to build it ourselves. Heck, they even went as far as allowing us to build our own fences. And many more examples of this can be given. So where they can give us animals in customizable habitats, they will ( looking at Meerkats and Prairie dogs).
The aforementioned reason is also why I think that
if we get birds, they won't come in exhibits. Not only would we have had them by now (as, after all, looped animations are a hell of a lot easier than the other mechanics like diving and digging we got so far), but it just doesn't fit with their overall approach to habitat design in the game.
So I agree with
@Mjmannella here. It's a balancing act when it comes to expressing your wishes to a developer. On one hand, you are less limited and can think outside the box, but on the other hand you need to also take into account the decisions and the direction the developers have decided to go in. You have a little grey area, and that's where you operate, to quote the great Tony Stark. If you stick to that, your wishes become more approachable and you're much more likely to actually see them come into fruition.
Overall, I think as
@random goat said, the chances of us getting smaller active mammals than a meerkat or a prairie dog is unlikely. So I'd say it's much better to let the meta wishlists remain in the realms of what is possible and what we know Frontier has decided upon. There's wiggle room, but not much, and I'd personally respect that. It's a game of give and take, and if you give a little, your wishes are much more likely to be considered.
