There is like 153 animals in the game right now. People can use the trading centre not only to sell animals, but also to store future breeders. If you imagine a person running big zoos they will no doubt have many species. I find my trade centre is often maxed out with around 180 "future breeder" animals, descended from unrelated parentage. I then use the last 20 slots for selling animals. Despite the 200 slot limit (a huge increase from the original 50) I feel the space requirements prevent me from expanding onto breeding other species. I find myself breeding so many different species, but it's not enough storage to expand onto more species anymore.

I breed a lot of the rarer species, so it's very hard for me to get new breeding stock from the market. I tend to have to create multiple good bloodlines from frontier sold animals. I have made 4 good arctic wolf blood lines for instance, but to prevent inbreeding I must also keep a good chunk of the individuals produced... It means my storage is packed with lots of good artic wolves. I need them because I use them for future breeding projects and I cannot sacrifice them as if I do I have to breed a completely new line from scratch as barely anyone sells good ones anyway.

Maybe this is the reason some species are so rare on the market? It makes sense for players to prioritize, especially considering how limited storage space is.