Less of a nerf, more of an unnecessary shift in his power.
There was a significant change in the past that basically put a lot of his early game power away and gave Renekton more scaling, taking away his lane bullying strength and making him not fall off as hard late. Specifically, his ultimate's power.
The problem is that that was the entire point of playing Renekton in the first place. Renekton was the champion you used to dominate the hell out of the early game, sacrificing your late game for screwing the hell out of the enemy top laner's. When Riot moved a lot of Renekton's early game power into scaling well, Renekton fell out of his niche.