You would probably have to consult AASHO or the BPR from about 1957 to get the actual answer.
My suspicion is that I-85 and I-285 have a 2+mile common section in the SW section of town near the airport, which would make a good place to terminate I-285, I suspect that could had contributed.
There is no common section with I-75. Plus I-285 is approximately evenly bisected between both I-85 interchanges. The two "sides" of I-285 between the I-75 interchanges are not even as far as distance is concerned. The eastern arc is much longer than the western arc.