Mr H l think it is because of the completely political way districts are drawn. Modern technology lets state legislatures pack a few districts such that the opposing party will win those districts by 75% to 85% and the remaining majority of districts with enough voters to assure "their" party candidate wins but by only 58% to 65%. Either way all districts are skewed so expertly that most people in those districts are very biased one way or the other.
In the last election l believe 13 of the 14 GA districts were decided by a vote spread of 24% or more. I long for the day for truly competitive districts where the vote spread is in the single digits. When we get to that place there will be no need for term limits...we will have terms limits called elections.
It has been said that once the voters chose their representatives, today the representatives choose their voters.