The meta is a carefully crafted organism that is constantly evolving.
The idea is that it makes you farm as many heroes as possible, so you spend as much money as possible.
It does this by slightly changing here and there so that one day a hero is meta, the next day they're not. They can do this through buffs or nerfs or through other heroes outperforming or under performing against a certain hero as they progress through the levels.
Case in point, Sidious. He wasn't nerfed, he simply lost power relative to the rest of the pack. He fell behind while others soldiered on.