League of Legends (LoL) Question: Why is riot reworking many champions recently?

is simple, riot make the champions who no one play more attractive, people buy these champions, see they are op, buy skins, riot nerf they, riot give a lot of money.
So in short words, for money.

Riot has 3 teams for "big reworks", which are the ones handling the really outdated champions. IDK if you've been playing LoL long enough, but champs like Taric, Galio, Poppy and a bunch more had really awful kits that needed a full overhaul for them to be possible to balance. Two of those teams often work on single champion reworks (they're working on Urgot and Evelynn right now) while the other works on class updates (last one was tank update a few patches ago, IDK what's next).

On why reworks are needed, the list is pretty long. Riot does lots of blogs all of the time on the topic, and there are lots of different reasons for reworks. Unfixable balance issues, game health concerns, "boring" kits (technically a game health concern and balance issue), lack of depth, are some reasons they often cite.

People complaining or not complaining is not a good metric either. I mean, besides the fact that YOU haven't seen people complaining about them, and you clearly haven't read all that there is out there, most often people complaining are nothing more than minorities, not representing the will of the majority.

That said, most of the recent reworks just seem like the coincidence between the regular 2-4 "full" reworks per year cycle (eve and urgot after warwick and galio) and some minor work that has been on the works for over a year (rek'sai and heimer). So yeah, seems like a coincidence on top of the regular schedule.

On the particular cases you cited:
- Heimerdinger has a huge disparity on melee vs ranged interactions (melee champions are too weak against him).
- Rek'Sai's ult is too powerful on coordinated teams and too weak on uncoordinated soloqueue (plus her kit has other minor issues).
- I'm not sure about Evelynn, but her kit seems outdated in general and she always drifts from unhealtily OP to borderline unplayable, which is a clear indicator that she needs a rework. I think she's getting a full rework instead of a small rework like rek and heimer because her visuals are also very outdated.

It is much easier than making a new one from scratch. Plus everyone needs a refresh ;)