Considering Riot usually looks at many things, one being the last time a Champ had a skin and another does it fit into the theme they are working on is usually how they make most skins. Cassiopeia can fit into a lot of themes as the community has made many of them out there, so Riot has plenty of ideas to choose from. it's pretty likely she will get one soon since the mage update is still fairly recent.
I mean what's the point of making a skin for Urgot if he only has a 0.6 play rate?
BECAUSE HE IS AWESOME Actually he is getting a VGU soon enough in the next year or two. So, they will be making new skins for him again regardless of play as that's how they do that. We can always expect a new skin shortly after the champion has been fully updated.