Champion are selected for skins based on two major factors:
Number 1: How long has it been since they have had a skin made. Champions who haven't had a skin in a while are more likely to have a skin made for them once a good concept is selected for them; this does not mean newer champions or those who have received skins recently are out of contention to get more skins. However champions with upcoming reworks are out of contention as Riot doesn't want to release a skin and then potentially have it change significantly if the model change is substantial. (side not Yorick hasn't had a skin in several years)
Number 2: Does a champion skin fit into a particular thematic. For example, if Riot came up with an awesome Zombie Zed skin, they wouldn't want to release it in July, they would wait until the Harrowing (October) to release it at a time that makes sense. This is the reason why while they had Pool Pary Zac done in late winter/early spring, they waited until the pool party even to release it.
While these are how the choose how to make skins, certain skins take far longer to make depending on the champion. Zyra for example needs 4 new models; Champion model, thornspitter model, whiplash model, passive state model. As well as changed particles. Zed would require his shadows to match any new models made for a skin for him. Champions that require multiple/complex models/particles tend have fewer skin released because of the scope required.
TL;DR - Any champ that does not have a rework planned ahead of them can get a new skin. Champions who haven't had one released in a while are higher on the priority list, though this doesn't mean champion on the list won't get one over them. If a skin matches a theme (Harrowing, Pool Party, Snowdown) they may hold onto the skin until it's the time of that thematic. Skin difficulty is a factor, though not one that auto-disqualifies a champ from getting a skin, it just may take longer for them to get it.