Champion Reworks

I'd like to discuss champion reworks. In general why do some champs get chosen, while others get ignored. One of the easiest examples of just how confusing this can be, is the recent Shen rework. Personally, I still don't understand why he needed a rework, I saw him all the time in high-level play, especially in the LCS; while someone like Warwick is virtually never played and has a really weak kit. This sort of thing can be continued to the other rarities like Urgot, Yorick, and Taric, who I also never see. Although Yorick and Taric will finally get there due around the mage update; and that mage update is something else in its own rite, I still don't agree with why those six where chosen over others.