(Disclaimer, I am not a game designer so take everything I say with a grain of salt) I don't think they were showed higher priority. They had much simpler reworks than the levels proposed for Poppy and Yorick. They are both Sion level projects that are probably already months in the process of being reworked, but it takes a lot of time and resources to completely re"Launch" a champion rather than just change their abilities a little bit. For Ryze, he received new spell visuals and an altered E but changing Q to a skillshot wouldn't be that challenging and all of his animations are the same. For Ashe they introduced a new Q entirely, slightly changed her W, and changed the E to be global and stack charges. Again, none of her animations were changed, she has the same model, and the hardest thing to work out was her Q and then coding on her "crit".
As for Poppy, her E is the only thing that is known to be salvaged from her old kit. Similar to how Sion only keeping his W from his old kit (and the passive on his old E). She is up for an entire kit rework, a visual overhaul including her model, skins, and artwork, a new voiceover, and new lore. Poppy is a BIG project, whereas Ryze and Ashe were small projects that could be completed rather quickly. The resources that you would have to allocate to complete a Sion level project in the same amount of time that you can complete an Ashe level project would be enormous. So instead the team is taking their time to complete Poppy by allocating smaller amounts of resources over a longer period of time, so they can work on smaller things at the same time.
As for Yorick. I forget if his rework was iceboxed or not. His was announced a LONG time ago, but he is a problem and Riot has acknowledged that. If UOL play him some more in competitive, he might get pushed forward a little bit, but right now, he doesn't see enough play for his "toxic" kit to be a problem.