Why has Kennen not had a new skin since 2011?
Currently only 4 champs have not had a new skin since 2011:

From those 4 it has been stated that Yorick and Taric will get new skins, but only after their reworks, so they are excused for now, meaning we have 2 left.
Sion is excusable since he had that massive overhaul recently with completely new models and all, so he is excused as well.
Kennen, however, has no such excuse. It has been 4 years since he has had a new skin, and for a champion that is perfectly fine there is 0 excuse as to why he has yet to have a new skin. Are there even any skins planned for him or did you guys just forget about him completely?
style of Yordles. Like Tristana was and Poppy is about to be. I think kennen's kit is fine so they will likely give him visual tweaks only. This is my only guess as to why he hasn't had a skin.