Several reasons - firstly, she's an unknown commodity; she doesn't have any kind of name recognition. Secondly, McKinney has a huge organization which was built around prior campaigns for her US Congressional runs as well as her stints in the State House. Third is name recognition - McKinney and her father are still very popular folks in metro Atlanta; even though she is an embarrasment and her dad (a former Atlanta cop and current Georgia state representative) is just plain crazy.
What it boils down to is that in many quarters in the less affluent portions of her district, she is seen as a hero who has "taken on the man." They are about as dim as she is, and if they outnumber more rational folks at the polls, she's back in - none of the GOP candidates have a snowball's chance in h@ll of beating her in the general election.
Bingo! - bump. The man knows his local poly-tics.