I've never really understood it, Barney. I grew up in Powell/Heiskell, and Karns really wasn't much different, as far as the schools are concerned. Just your typical middle-class suburban school, and certainly not a "failing" school by any means. My niece graduated from there a few years ago, and what I saw wasn't much different from what I experienced growing up in that area. The proximity of GCA and HVA definitely doesn't help, but the Karns zone has shifted more to the east and contains the entire Emory Road corridor west of Clinton Hwy, which used to be part of the Powell zone.
Even before GCA and HVA opened, Karns was a doormat on the gridiron, and I've never really understood why. They've played in the largest classification for almost their entire history, which hasn't helped, but they've been competitive in a lot of other sports. HTV has mentioned this before, but I know that their baseball program hasn't exactly encouraged their players to compete in multiple sports. Not sure how much that has played into their lack of football success, but it could be a factor.