imo the reason theyre paying more attention to galio is 2 fold. 1- he got his rework, so obviously they were focusing on him at that point. But after worlds because of the dominance of full tank galio they reworked his ratios (to reward him for building ap, and to make building 100% tank less rewarding), making him into more of an ap bruiser/ap diver than a tank, and they are still paying attention to him to try and tune that correctly.
On the other hand, the champs you brought up, voli, udyr, skarner, all have much older kits (like pre-rework galio had) with less counterplay for the opponent or skill expression for those champs, i think skarner has been worked on most recently of the 3, and because their kits are like that riot is waiting til they can put the resources towards reworks/vgus for those champs to modernize them rather than making changes to their existing, outdated kits that are not as in line w/ league of legends today.
I do understand, especially if you are a main or someone who really loves one of these older champs, the frustration of seeing more popular champions get worked on multiple times while your champ just kind of sits there, (Tbh i would LOVE too see a jax VGU/rework, 276k mastery on jax 1st champ i mained, because he's too simple imo, not saying he's bad but he's pretty much a stat check). But riot has been (slowly but surely) making progress toward updating these older champs, and eventually they will get to these meme team champs that have been left in the gutter (I hope).