"The loss of the Atlanta battlefields is, I think, the single greatest tragedy in the history of Civil War battlefield preservation."
Loss or tragedy, the Atlanta battlefields have been gone long before we were born. In some places of Atlanta, the suburbs, one can hardly recognize some area after a twenty year visit. Atlantans and Georgians did not preserve Union victories. Any battlefield saved in Georgia over 10 acres is probably a Confederate victory. It was probably too hard to fathom that the Confederates lost the war.
There are only 2 save battlefields out of dozens so even though your point is well made I may disagree.
Most of the battlefields lay along the railroad to Chattanooga and hwy 41 (Dixie Hwy) which has seen development since the early 1900s. Then add in the construction of I-75 (I have heard its the heaviest volume interstate in the states) along the same route which added to the development. Kennesaw was saved because the government appropriated enough funds for 1 park and that was between Peachtree Creek and Kennesaw. Peachtree Creek was already in a residential area by the late 1800s and Kennesaw was in the boonies. Plus you can't develop a mountain, really. Then add the fact that some of Kennesaw had already been purchased by veteran organizations.
Aside from these:
Pickett's Mill I don't know the story of but it could be likely that there was more interest in it due to the Confed victory, I don't know.
Other than those the Cascade Nature Preserve is on a small part of the battle of Utoy Creek by mere coincidence (Confederate victory).
Resaca has been recently preserved (Battle was basically a draw).
Rocky Face Ridge has been somewhat preserved due to the roughness of the terrain being unsuitable for development (Confed victory)
Dug Gap has been saved (Confed victory)
Griswoldville has been saved (Federal victory)
Allatoona Pass (Federal victory)
Brown's Mill (Confed victory)
Parts of the River Line (Confed withdrawal)
Lovejoy Station (Confed victory but I think the land they saved turned out to not even be the actual battlefield)
Major battlefields lost:
Tunnel Hill (Confed)
Lay's Ferry (Federal)
Rome Crossroads (Confed)
Cassville (Confed withdrawal)
Dallas (Federal)
New Hope Church (Confed)
Ruff's Mill (Confed)
Smyrna Campground (Confed)
River crossings (Federal)
Moore's Mill (skirmish, Confed withdrawal)
Peachtree Creek (Federal)
Atlanta (Federal)
Ezra Church (Federal)
Atlanta defenses except Fort Walker
Sunshine Church (Confed)
Jonesboro (Federal)
What else is there?