Because Iommi is a much greater rock guitarist.
Well, going by George's criteria:
Importance - Berry by far. Iommi is the foundation for the whole metal style, but Berry is the foundation for ROCK guitar itself. It was Chuck, more than anyone else, who made electric guitar the lead instrument in rock. The entire concept of the 'guitar god' itself certainly owes a healthy amount to Chuck.
Innovation - I say Berry because he helped to innovate a certain 'riff rock' kind of sound that would serve as a template for pretty much a huge majority of rock guitarists to follow (think AC/DC for instance), and on a broader basis than Iommi.
However, Iommi (along with Butler) innovated the entire heavy metal sound. The thick, weighty downtuned pounding that became the metal trademark.
Respect From Other Guitarists - Berry by a comfortable amount. Soooooo many have praised sang his praises for opening the doors to guitar-dominated rock in general.
Influence - Berry by a very small amount. Has profoundly influenced giants from Eric Clapton to Keith Richards to Jeff Beck and far beyond.
Legendary Status - Tied. Pretty much a no-brainer that this is impossible.
Creativity - Iommi. Planet Caravan.
Versatility - Iommi. Pretty obvious.
Musical Depth And Expression - I have no clue on how George thinks this can be determined, but neither one of them really excel, so I say a tie.
Live Presence - Berry by a landslide.
Improv Skills - Iommi
Originality - Iommi by a little. While there were a few guitar records similar to Chuck before his breakthrough, there were VERY few guitarists playing like Iommi prior to his arrival on the scene. Maybe Leigh Stephens from Blue Cheer, and to a lesser extent Tony Hill from High Tide, but Iommi was more original.