Riot doesn't rework based solely on priority, they also rework based on the scope of the project and whether or not they have some ideas for what they want to do with the champion.
For example lets compare the Fiora rework to the Yorick one. Yorick was a far larger issue and had been for far longer, however Riot was less sure of what they wanted to do for Yorick. Was he a minionmancer? Was he some sort of necromancer? Did base him around his actual gameplay of a bruiser with heavy lane dominance and sustain? On top of that if they were preserving his undead that meant he would require far more assets than most champions, which means far more work (Models, Textures, Animations, Ability FX, Sound FX, possible VO)
On the other hand Riot had a very clear idea of what Fiora was meant to be, the grand duelist, as such they had a far easier time figuring out what they wanted to do with her, and she required far fewer assets.
Now lets look at the 3 you mentioned: Morde, Aatrox, and Ryze.
Morde suffers from many of the issues Yorick did. He has a number of paths that he can be taken down, if they plan to keep his whole necromancer shtick then he will most likely take a ton of extra assets, he isn't popular, and a partial rework wont cut it by any means. Thus he is going to take a ton of work and needs to be completely rebuilt from the ground up.
Aatrox is in a slightly better state. First of all his current gameplay state isn't actually bad. In fact thanks to his mini-rework he is probably in the best state he has ever been. The reason he is currently being reworked over Morde is because he is a much smaller scope rework. While he will be getting some things like a new VO to match his new kit/lore, his overall work is going to be very low for a VGU. Part of the reason Riot put him off was because they were deciding which of two paths to take him down, the Bloodthirsty god his gameplay/visuals screamed or the god who commands and inspires warriors like his lore portrayed him as.
Ryze. Currently his 6th? rework is in progress. Why? Well first of all it is because Riot knows Ryze's theme. He is the machine gun mage. Short range, some CC, etc. They also have his lore and personality pinned down. Also going to clear up some things about his reworks. Keep in mind his first three reworks were back in season 1. His 4th rework was in 5.8, and was somewhat of a hold over one, like how Aatrox got a mini hold over rework. It was also to help his visuals a bit, no model update, but his skins did get texture updates. Which takes us to the 5th rework, the big one that we are currently sitting on. Unlike his other reworks (GU) this was a full VGU update and was done to try and bring both his kit and visuals up to modern league standards. Unfortunately is has proven to be difficult to balance between high and low tier play.
Whereas Morde and Aatrox can be played at low tiers (aka in over half of all games) Ryze has to have so much power sucked out of his kit that he is only viable in high tiers where team coordination makes up for it. Even then though he rarely gets played in normal league, and really only gets played in pro games where everyone is in voice chat. This means that whereas Morde and Aatrox can be played with success in most league games, Ryze cannot. Also keep in mind that this "rework" is no more than one of the many small ones that have been cropping up recently, such as the Ahri one. They don't use new assets, they are purely mechanic changes to existing abilities to help shift the champion's power around (which is different from buffs/nerfs which change the values of abilities.)
Basically Ryze's 6th rework is not actually delaying Morde/Aatrox's reworks in any way, it is something Riot is doing to help a champion with major issues fit into league, and I would guess that like the Ahri changes it is pretty much entirely the work of a single person, rather than a whole team like actual GU/VGUs.