And that reason is "supporting" cast. I'm not saying Jennings is better (I don't believe that he is), but you can't say Hill's teams are normally in the Top 3 because of George Hill. He's the 5th best starter here and was a backup in San Antonio.
The biggest knock on Hill is that he's not a strong passer. He had some great passing games earlier in the season, but they are few and far between of late. Sometimes he telegraphs his passes and they are picked off for easy fastbreak points. More often, though, he just lobs them, meaning that when his teammate receives them, they aren't as open as they could be - he allows the defense to recover.