Popular champions = more ppl play them.
More ppl play them = more ppl buy skins on them
More skins = more money
Ivern, A-sol and Lissandra are currently the least played champions on all regions in ranked.
I dont think riot is happy to spend time and money on the development of skins for such champions if only 70k people play ivern and 100k people play A-sol each month.
For reference, The most popular champion, Tristana, has been played over 2 million times since last month.
Tali could get a new skin, she's not so unpopular now. But A-sol? nah