Roadhog wasn't overpowered, but his skill floor for being effective and getting kills was much lower than that of the DPS characters that should be focusing him down. So when low-ranked Hogs faced off against low-rank DPS, the DPS would either just get hooked endlessly all match or get frustrated and give up on trying to shoot him in favor of chasing down a Genji or some nonsense.
Like, ideally people would just learn basic positioning and the Roadhog problem would've disappeared, but people think they're already fantastic at the game and would rather see a "cheap" character get nerfed than have their assumptions challenged. So I think, at some level, the Roadhog nerf was Blizzard balancing around players' unwillingness to learn the game, rather than actual mechanical impact and efficacy.