Jason,
Great question and I have pondered as well in the past. I am thinking that Minor league players back in the day were more apt to sign a ball, or a scorecard, piece of paper, etc. Also, I think when you speak of the series T209 and T210, and their relative scarcity, I bet there wasn't many of the player cards necessarily in their home town area. I think of how few there are today, and perhaps those issues were in packs at different locations all over the East/South. I would even venture a guess, that some of the players had no clue they even had a card.
I think a good clue may be to look at the Major league guys in T206 and how scant few auto's there are of those players. Then deduce it down to Southern Association, and even further scarcity of a T210 compared to T206 and you would have very little chance of ever finding one.
For me, I just don't think those T209's, T210's were readily available for say a kid to run up to a Southern Leaguer and ask for auto on the T210 as an example. My guess is the cards might have resided more in other locations.
I would love to find one, own one, but when I can only find in 15+ years of searching for a Harry Cohen from the Nashville Vols - 1910 T210-8, 2-3 examples known, then I am guessing Mr. Cohen didn't ever see his own card either.
