Well let's go through it:
His power's pretty much kept in check by the virtue of him not being dumb and knowing he isn't the most powerful being on Runeterra. He was imprisoned once, he probably learned from it, and so he's being more careful now.
He isn't a mindless monster, he's cunning, intelligent, and sinister. While he could just go on a rampage and kill loads of people that's probably not how he actually likes doing things. He likes to toy with his prey, to break them before eating them.
He's the HARBINGER OF DOOM!!! What's the doom? Er...no one really knows. But it's DOOM so it's gotta be big right? Well who knows really. Until they actually define who Fiddlesticks is without the IoW, we can't really say about to him...at all. We can speculate and conjure up theories, but all while lacking knowledge of what the hell DOOM is when it comes to Fiddle.
He spent a long ass time stuck his his own grave, despite being in his mindless monster-ish state. This was without supervision for almost the entirety of that time. I'm pretty sure Swain has a handle on how to keep Sion in check.
She's Shurima bound; doesn't really have a reason to leave the Sai as long as there's adequate prey, and there's plenty of Shurimans still milling about for her to gnaw on.
The Shadow Isles champs are pretty much bound by the Black Mist and the will of the king, providing he's even the one commanding the un-living legions, so it's basically not in their hands 100% of the time whether they kill things.
Essentially if you want to answer this for everyone, just look at the champion and think: what benefit is there to not going in guns blazing and wrecking everything for this fellow? Also ask if they're mindless killers or not, and what in their lore would be a contributing factor to them not wrecking everything. There's some interesting perspectives you can get through conjecture and theory.