@emb is correct. it helps to prevent huge segments of sentences being in hiragana only and having to waste brainpower parsing the n/adj/conj from the particles.

as for not writing them in kanji, many animal names (especially those from more foreign/distant habitat from Japan perspective) use kanji that are outside the jōyō list and thus most native speakers are not expected to know them. often these kanji were originally invented in ancient China specifically for those animals and assigned pronunciations to approximate preexisting foreign names from languages of the animals original habitats region or languages of the traders who brought the animals to China.

additionally, being invented kanji, they sometimes have a huge number of strokes. this is no problem for a highly educated young person reading a book at close distance. for an elderly person with deteriorating vision trying to read the name on a sign (at a zoo, for instance) from meters away, the kanji may as well be a QR code. by the same token, some medical providers also have exterior signs with kana replacing one or more characters in a kanji term. this is not standard, but it can be thought of as consideration for those with vision difficulties.

furthermore, the more complicated kanji are learned in high school (and some in college and just adult life) but compulsory schooling is only up to jr high/middle school. and many elderly folks could have grown up in the postwar era when many schools were not yet rebuilt.

https://youtube.com/shorts/zugG5AJBFDY

Kyota Ko did this video as an explainer. They tried to eradicate more confusing and irregular Kanji readings because they often make no sense and you don't deal with all animals every day. So just write them in katakana and that makes it obvious enough you're dealing with an animal and you don't need subtitles for it.

The animals that regularly get a Kanji today are common AND their Kanji has - in the logic of Japanese - unambiguous readings (e.g. 猫, 犬, 像, etc.).

Don't ever encourage Japanese to do more with Kanji. Won't anybody think of the gaijin?

I don't know so I'm speculating but

frequency

I'd assume that's the issue. How often do you talk about crocodiles in your everyday life? Now imagine you'd have to decipher the kanji. It's simply more convenient because people can't be expected to remember the kanji for a billion animals.

Feels like it's kind of parallel to how we'd write specific species names in Latin and italics. Convention, that also serves to make things stick out.

Maybe?

In Japanese, when distinguishing between biological organisms, it is customary to write them in katakana.

For example, the biological organism referred to as "人間 / human" is always written as "Hito / ヒト" in katakana.

However, in the case of food products, there is sometimes a convention of writing them in kanji.

For example, in the case of the character for "fish /魚 ," the meaning of the fish is embedded in the kanji (example: 鰻 / eel), which can make it seem more appetizing.