The skin being delayed is unintentional. I think it's mostly due to timing and Riot finishing up some things with the skin.
About the timing: most things that are said to "release later in the patch" usually release about 2-3 days afterwards. So if the patch comes out on Tuesday, then those things are likely to come out on Thursday/Friday. And guess what happened this Thur/Fri? Thanksgiving and Black Friday. Riot probably knows this is a really busy time for people, with everyone traveling/spending time with family/etc, so it's not a good time to release it, everyone's too distracted.
Also, another contributing factor: Think back to all the other Ultimate skins. Every one of them had a big promotional video or comic that came with its release to hype it up and showcase it in all its true glory. That's exactly what Riot is doing with Elementalist Lux. High quality CG videos and artworks take time, and what they were working on probably wasn't finished yet. And with Thanksgiving, obviously Rioters have families too so they took time off for family.
I'm willing to bet 100% that Riot will finish up the promo and release the skin on Monday. It's after all the holiday craziness has died down, everyone's back to work and no ones too busy or distracted so the skin will get the attention it deserves upon its release.