The others intrested in the Isakson seat--Former GOP head Chuck Clay, Barr, and Lamut either do not have sufficient name recognition (Lamut and Clay--who ran for Lieuteneant Governor 4 years ago, but is not from that district, and there is much turnover in that district) or run counter to the socially moderate, fiscal conservative electorate in Isakson's district.
Isakson is a country club Republican and won overwhelmingly in the district, which is composed of former Northeasterners and Midwesterners. They won't elect Barr, who did better in his former district (to the Alabama line, where you had a lot of gun toting rural white Democrats).