Crackpot theory.
Maddie survived as a psychic ghost after Genosha and lingered until the future where baby Cable arrives. Using part of phoenix force, she willed herself a new physical body, that lasted just long enough so she could raise him into being able to survive on his own.
Essentialy taking a bit from the X-man series as inspiration.
Which then explains why Cable would have never been able to safe her, since her death is required for his own existence.