Why is the area between Johns Creek, GA and Peachtree Corners, GA so Republican?

[Del Tachi]

This area contains Country Club of the South, which is an extremely high-end area (multi-million dollar houses) and I personally think has more of an Old South vibe than some of the other country club neighborhoods in the metro area. The D trendlines here are the same as all the surrounding areas, but this area was 80-90% to begin with a few years ago as opposed to 60-70% R, so it hasn't gotten to the point of flipping yet.

Interesting. Moreso than Buckhead? I thought the residential areas near Buckhead area were very "Old South."

The City of Atlanta is very, very, very much the epitome of the New South.  Buckhead may be full of the city's "old money" but Atlanta has always considered itself on the leading edge of other Southern metroes.  Y'know, Henry Grady and all that

[Del Tachi]

To answer OP, yes this corner of Fulton County is very wealthy and white.  However, that's kinda begging the question isn't it?  Why is it wealthier and Whiter than neighboring areas of Roswell, Alpharetta and Gwinnett?

The answer is mostly inline with Sol's earlier comment upthread.  The parts of Metro Atlanta that have seen the biggest D trends are quickly diversifying areas along the region's major arterial roads (like GA 400 and Interstate 85.)  That's because these areas developed first (mostly in the 1950s-80s), so the housing is reaching the end of its usable life and ready to be renovated, upgraded or rezoned into higher density development; this activity brings newer residents.  That this corner of Fulton is relatively remote means little demand for newer development. 

This paper is a good read on the spatial economics of residential redevelopment.  Theoretically, cities grow out from a central point so the newest, most desirable housing should always be on the urban edge.  However, renovation in old age housing stock near the city center reestablishes these properties' amenity values - thus drawing rich gentrifiers back into the city.  You can see this on display in the City of Atlanta, where pretty much all of the "intown" neighborhoods of mostly pre-WWII housing vintage (i.e., Virginia Highland, O4W, Inman Park, Morningside-Lenox Park) have had their housing reestablished.   The epicenter of "gentrification" in Metro Atlanta has now moved away from these neighborhoods further away from the center; young families are buying up mid-century ranches in Tucker and  Stone Mountain to take advantage of the rent gaps their. 

I don't really believe gentrification to be a social or political phenomenon, but really just an economic reality caused by the need for residential buildings to periodically be updated/renovated (most American housing has somewhere between a 50-70 year "life cycle.")  In the U.S., this pattern of gentrification generates a lot of conflict and attention but its because our urban landscapes are defined by big, discrete waves of residential development that result in proximate housing being more or less the same age (contrast with Europe, where you have cities of ancient vintage that have thoroughly mixed housing stock) not "muh rashist White ppl, KKK red lining machine go brrrrr"