Zyra has been stated as being one of the more difficult characters to make skins for due to the sheer amount of models to work on, in between her, her pets, and her abilities.
Illaoi is in the list for the same reason, hence why Riot did the experiment of letting the playerbase vote for her next skin (and that went “beautifully” with idiots crying about “edgy kids” when best skin won both popular and Illaoi players votes). As well as Kayn, which is why the team went “fuck it” and made a legendary for his first non-release skin seeing as they were going to put in the amount of work anyways.
Both her below average play rate and the sheer amount of work it would take makes a skin for Zyra a rare occurrence.