why only animals

In the first season alone, there were a time span of roughly 400 million years. Considering we only saw about 15 anomalies in the "spaghetti junction", that would be an average of roughly 26.5 million years per anomaly.

Humans have only been around for about 2 million years, so by pure chance, an anomaly opening to any other point in human history is seriously unlikely.

If we move on to season 2, in the first episode there was a portal to a distant future. How distant we don't know, but considering a bat had grown to 15 times their current size and evolving super-human intelligence, I'd say about 100-150 million years wouldn't be too far off the ball-park.

That gives a time span of 500-550 million years divided among the 15 anomalies in the junction, or an average of about 33-37 million years per anomaly (given an even spacing).

And then we have episode 2 of season 2, that goes way, way back. We're talking something like 750 million years... Well, you're starting to get the idea, right? For a portal to open to only 70-80 years ago would be an extremely low chance...

Then again, the show writers clearly have no idea about time mechanics and seem to see the past, the present and the future flowing at the same rate, even though the show is about time travel. Helena is always at the same relative point in her time as everyone is in the present, she only travels through anomalies that would take her to the exact relative moment from when she left, which makes no sense at all... So yeah, there's nothing really stopping them from writing in nazis or egyptians... or as they did: knights.

I think in part, it has to do with the company who does the animation. Impossible Pictures were the creators of Walking with... (Beasts, Dinosaurs, & Monsters) and that is kind of their trademark. They were asked to animate the dinosaurs, pterosaurs and entelodont for the 2001 miniseries The Lost World.