I think a fairly large factor in certain Champions not getting tons of skins is something to do with their overall design, and how it can limit what Riot can do with that Champion.
Using Skarner as an example, he does have a fair few skins already - Sandscourge, which presents him as more of an actual, overgrown scorpion, Earthrune, which turns him into more of a wild, fae sentinel of moss and stone, Battlecast Alpha, which turns him into a mechanized siege machine in Viktor's robotic army, and Guardian of the Sands, which makes him a Shuriman tomb sentinel.
And, sure, there could be other unique ways to spin how the Crystal Vanguard looks - perhaps giving him a Bilgewater thematic and turning him into something more like a lobster, or making him a life-seeking abberation of the Void...But compelling thematics have to be carefully considered with Champions like Skarner, especially if they wind up ultimately putting more work on Riot's plate when they inevitably look to him for any measure of Gameplay update.
Likewise, Volibear has been stated to be on the table for a future, small-scale Gameplay Update, whenever they may find the need and direction to work on him, so making new skins for him would be a risk due to there being the potential of adding more work onto future updates.
Cassiopeia is somewhat of a unique case, though, as her gameplay seems to have been cemented since the Mage Update two years ago, but she also very recently got her Eternum Skin. Between that and her thematic (being a medusa/serpentine creature) being relatively difficult to work with in terms of good, interesting thematics, it's fair to assume Cass may be waiting a fair enough while to get a new skin.
The same rule as before (difficult thematic/design) can apply to Camille - she's humanoid, but her blade legs kind of make it pretty difficult to some cohesive concepts.
Jinx, on the other hand...She's a crazy, wiry girl with long braids and hugeass guns, with very specific functions. The amount of shit they can do with her is immense given...Well, she's fuckin' crazy. Anything can work on her if it's not super serious.
And just look at her skins - Mafia, Firecracker, (zombie) Slayer, Star Guardian, Ambitious Elf. They're all vibrant, wild, and varied. It'd take a fair bit more effort to turn some of those more light-hearted skin thematics (Firecracker/Lunar Revel, Christmas, Star Guardian) into an actual skin for someone as austere as Camille.
If we're going to acknowledge the fact that some Champions aren't going to get skins for a long while, we have to at least try to understand why they aren't - and good god are there a number of reasons for a fair few Champions.
Besides - Riot typically has some of their skins planned well in advance, so who knows? Maybe Camille is gonna get her second skin in a few patches, and perhaps Skarner might see gameplay changes and a skin immediately following them, like Swain did.
teaser (yup, she's getting a new skin while 
still has only one). If you are a main of an unpopular champion, you should get comfortable with the skins your champion got, because that's all you're ever gonna get, for a long long time.
) to make up for lower revenues.
