If a rework is ready to ship, there's no real reason to hold it back, and some reworks likely got released before others because one or more designers happened to have good ideas for changes at the right time. Alongside that, though, champions like
and
have been worked on for quite some time, and at that stage it depends on the champion itself. Ryze's problem boiled down to just gameplay, where the lack of peaks and valleys in his power, as well as 100% complete reliability, made him extremely flat and impossible to buff, and so it's likely his rework ended up being a matter of parceling his power better. Gangplank, on the other hand, has some deeper problems: his kit lacks gameplay cohesion, despite having a good running theme, and the most iconic parts of his kit (his Q crit and oranges) also run the risk of being extremely low on interaction. Rioters have mentioned that they're working on him, but it's likely his rework will take more time than Ryze's from start to finish, since they need to make him interesting and fun for everyone, while still keeping as many of the quirky bits of his kit as they can.
On the extreme end of the scale, some of the most urgent reworks are also the most difficult:
,
and
have problems that go way beyond even GP or
, and have been in desperate need of a rework for years, yet doing so would require
levels of work for each of them. All of them are probably being overhauled even now, except we'll only see the results in a year, or two or three. Seeing how quick partial reworks like the recent
,
and
changes had mixed results, I'd rather Riot took the time to change a champion comprehensively and in one go, while trying to make that champion the best it can be, than they tried to just fix the most broken bits and left them half-done.