the female fanbase of supernatural kept getting the female characters killed off
anytime a woman so much as looked at sam or dean the studio was flooded with hate mail demanding they kill off the person trying to steal their imaginary boyfriends!!
its why jo wasnt in season 3 or 4. the fanbase hated her for flirting with dean.
the fanbase were more accepting of mother figures for the boys and as they got older they also liked daughter figures like claire