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I couldn't figure it until having watched it again; but it turns out that there is enough to know why his mother abandoned him. Considering the times, the situation and the few actions of Moorehead as his mother, I think I can safely assume that Kane's birth father is abusive, likely alcoholic; and his mother gave him up to insure he is safe from his father and the especially the people that they both became, the life they both sunk into; essentially her acquiescence.