@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.