Ekko just came out an already has a new skin. Bard has a chroma so I guess that's alright, but Azir hasn't gotten a skin since his release nor Kalista, and there are plenty of champions who haven't gotten a skin in forever like Taric, Brand, Swain, Kennen, Olaf, Nocturne, etc.
Obviously, I'm not Riot. However what I always assumed and made a lot of sense to me at least is that they are always coming up with new ideas for basically all the champs (probably thinking more about ones they personally play and see in game, not in a preference sense, but just in a awareness sense.) Then from there they would trim down that selection to the "good" ideas and try working on them and then only continue and release the ones they think are really good and that people would like.
i think riot likes to support the popular champions such as ekko,darius,ahri
They come up with skin lines(themes) and decide which champions fit those themes the best.
There are several factors: -When was the last time the champ got a skin?(a champ who just got a skin is less likely to get a new one) -How many skins does the champ have?(a champ with many skins gets less priority) -What kind of ideas the champion team has(ekko gets the skin because they simply liked him for the academy set) -What kind of events are coming(Rek'sai got the pool party skin early because of summer)
