Oglethorpe has an endowment much smaller (at least 5x less, 50+x less than Trinity) than any of the other ACS schools, so maybe that has something to do with it. Also could be a competition thing, since ACS is affiliated with Emory- even though it's a research institution they still compete for some students I think.
Berry is just really unique- largest campus in the country, nearly a billion dollar endowment, really strong environmental and animal science that don't typically go with liberal arts schools. The whole Martha Berry story is pretty incredible, cool place.

SAA likely not interested in adding anyone without some major academic clout. Could pull Huntington and LaGrange out of USASouth, pull Louisiana College and Belhaven from ASC, add MUW and then only need a couple more for a reasonably legitimate new conference that would save all those schools a lot of travel. Centenary would be a good geography fit but they may see it as a "downgrade" academically. Maybe Piedmont and/or Covenant out of USASouth or an NAIA pickup from SSAC that would probably fit better in D3 if the travel was better (UMobile, William Carey, Blue Mountain or the like)