Why do Shen and Poppy get Updated Splash Arts...

Because it wasn't a thing when he was reworked.

Because once you consider it, they might've just thought of doing it and it's part of a new regime for champion updates onward.

I think they will do that at some point. But Riot is busy with atm (and also a lot of balancing adjustments). Also there always is a consistent amount of skins in the works.

So i would expect it to be at some point, but there is so much more high priority stuff a the moment (Lunar Revel Event, Taric Rework, Mage Update, Yorick Rework, Mount Targon Event etc.)

It takes time and no one really expected it.

Reworked champions will get new splashes retroactively.

Because its a new thing that they previously didn't do. They updated their standards for VUs and thus every new VU is accompanied by updated(or at least adjusted) splash arts.

They will rework the old splash arts for other updated champions(and also the other outdated skin splash arts like they did with assassin yi for example),but that takes time and resources and the skin splashes for the new updates simply take priority over the old ones.