As with every other champion (Cassiopeia is no special snowflake, you know), they choose certain champions for certain themes because they feel like those champs just work the best with those themes.
-_-
Would Cassiopeia work in some (recently-released) skin lines/themes? Sure!
Would she be amongst the best possible candidates for that theme (at least for now)? Not necessarily.
I mean, forcing a champion onto a theme (instead of other champions who could fit into that theme better, for now) isn't a solution.
A lot of champions aren't the best possible best candidates for skins, and it shows with how we 'only' get 3 Snowdown skins for 3 very specific champions, but not for others (again, at least this or that time around).
-_-
Maybe they've been working on a new Cassiopeia skin and it's been in the works for more than a few months now. Maybe they haven't.
Whatever they are or aren't working on is (in most cases) what they think is the best in terms of Champion & skin theme crossing.
-_-
Sorry if I tried stressing my point a tad too many times. :P