The difference was programming style. Jacobs was the essence of Top 40 formatics, while Tom Rounds at KFRC had "discussions" with Drake about playing "White Rabbit" and the like. The Mount Tamalpais State Park Fantasy Faire concert proved Rounds was right for that market. Even though TR moved on to do the Miami Pop Festival, KFRC had been branded as being the station that would venture in to the harder rock areas successfully.
And I agree that KFRC was saved for longer than most AM Top 40's by the combination of terrain, lack of FM radios with AFC and low car penetration. But behind it all was some pretty good programming.
All true---though Jacobs deserves some credit for being savvy about rock as well. If there wasn't a hit single on an album by a major artist, Jacobs found tracks and programmed them. Some of it may have been that the artists themselves listened to KHJ---especially before KPPC and KMET came along---but Jacobs held firm. It wasn't really until after Jacobs left in mid-69 that the station would lean too bubblegum to be credible as a rock option for listeners. Fortunately, "The History of Rock and Roll" was one of Jacobs' last great acts.