They said that Eves skin was an exception and that they only did it because she was just too fitting for this skinline(and its also one of the reasons why the skin stayed at 750Rp,because they didn't want to give her new particles when she is on the VGU list).
Not to mention that this was done at a time where we could be happy to get one big VGU in a year,wheras now we get big VGUs at a much faster pace(as we'll likely get 4-5 VGUs this year).
And in Katarinas case it is safe to assume that her project skin was nearly finished already when they settled on the candidates for the assassin-rework(as the project skins came out shortly after they settled already on the assassin-reworks),thus Kata got onto the rework list only after they invested a lot of resoureces already into her Project skin.
We also don't know if Tier3-champs are even a thing or if they are not allowed to get skins.
Also even if those 3 champs were able to get skins,they are still worse choices for the dark star skin line than Kha'zix anyway.
I mean the dark stars are supposed to be alien abominations that devour entire planets,Amumu would have felt completely out of place and Volibear would have been extremely random.
The only one who could have fit the skin line is Nocturne,but again there is the fact that the dark stars devour planets,thus giving Kha'zix,a champion who eats others to grow stronger, is still more fitting.