There are 2 problems with traditional skins that you forget:
1.They cost resources.If riot did them for almost all reworked champs,then that would slow down the reworking process,which most people simply don't want,since there are so many champs that need a rework.
2.The traditional skins merely dress up the new version of the champ in the clothes of the old.
A traditional Warwick would still have this huge pipe looking out of his back,because that is something that is part of all of his skins.Traditional warwick would basically just have shorter ears and some armor on his arms,but otherwise look like the classic version.
And that is honestly not worth a skin.
Pretty much the same for yorick,who'd have a recolored outfit and maybe a shadow that covers half of his face in a traditional skin.