Mayer and especially Teel had the clearer paths to MLB than Montgomery, as the Sox control Duran, Rafaela, Abreu and Anthony all for the next 4 seasons minimum.
Teel is a big deal, as the Sox have nothing better than Wong above A ball (and maybe not even there).
Mayer, who’s only 4 months older than Montgomery, is already in AA and blocked by an oft-injured player who will definitely clear a path for Mayer by accident at some point.
As the White Sox best prospect is an MLB-ready shortstop named Colson Montgomery (no relation), I imagine that helped tip the scales in favor of Teel.