First of all, concept art. Speaks for itself why Kassadin would make for a great PROJECT skin.
Second, his last skin was released in October 2015.
Third, he has not seen any new skins, chromas, emotes or other cosmetics for soon to be 4 years. That's more time than Pyke and Kai'Sa have existed for.
Fourth, he currently has 5 skins in total.
With only the first three being purchasable at all times. And only ONE (Cosmic) that features a custom recall and VFX.
Kassadin fits perfectly with the PROJECT theme and is definitely a part of the list of champions who have not recieved a skin in a long time. And, if you remember, Riot did confirm that skins for forgotten and less popular champions will be developed.
So how the hell did Pyke, a champion who already got a skin this year, and who has existed for a shorter period of time than Kassadin's last skin, take the place?
And before some wise guy writes the "PoPuLaR AnD StRoNg ChAmPs SeLL BeTtER" speech...
I know Kassadin is no longer in his Season 2 99% ban rate status, but sweeping him under the rug and pretending like he does not exist is a very unprofessional move from Riot, not to mention disrespectful to Kassadin's dedicated mains.
And has Riot ever thought that if the skin is good, it might spike interest in the champion, which will increase play rate, which will increase popularity, which will increase the sales even further? If i remember, Udyr was a forgotten ugly boring mess of a champion before his ~~payed VGU~~ Ultimate skin was released. After that everyone and their nana became an Udyr player.
It's not about the champion Riot. It's about the quality, execution and most importantly, the advertisement for the skin. Something you seem to do very well with skins like K/DA, but half-ass on things like Papercraft.