Possibly because of the new unit, the Terran Marauder. This unit is incredibly useful as it slows the movement speed of units it attacks. This is highly useful against fending off Zealots, whom need to get in close to their target to actually strike them. Against Terran, this may be because the Marauders are in fact a widely used unit. Thus, because of its +10 damage to armored, they'd be hesitant to upgrade to Mechs.