Can someone explain something to me? How is that rock music singers of geographical areas with heavy accents do not seem to have singing voices with accents but country music singers are the exact opposite?

While on the highway today and scanning for music on the radio, every country singer had that twang, and then I landed on a Tom Petty song.  Tom Petty is from Florida but does not have a twang to his music. While he does have his own unique sound, it's not a twang. 

Or am I missing something?