Champion Reworks + Old Skin Splash Art

Dear Rito, I was simply wondering why whenever a Champion gets a Visual Update/Rework, their base splash art also gets redone to better reflect the champion, BUT, their existing skin splash arts do not get changed. This leaves the splash arts depicting the older version of the champion, but they look alot different. Most noteable examples of this are Sion, Gangplank, and some of the Twitch skins.

Is it because youre more focus on updating more champions and balancing rather then having an art team redo some of the splashes, would it just be too tedious of a thing to do, or do you simply not feel its needed?

Thanks in advance for any response