It has yet to be revealed , so John may not have known that this was Mary's worst fear. These Winchesters could be in a different universe from the Sam and Dean fans watched. Mary and John could also have had a mind wipe, meaning John wouldn't have remembered.
If he did know Mary's fear, it was probably the grief of losing his wife that made him forget. So much of John's hunting was driven by revenge, which is how most hunters get started, with someone they love being killed.
While Samuel Campbell didn't need to tell Mary about the family business that young, John was trying to
. His worst fear at that point was that a monster would kill his kids. And even though, according to lore, John didn't know Mary was a hunter, it's hard to escape that life without monsters trailing you — especially when the Campbells and Winchesters are legendary hunters.
Samuel didn't know it then, but he was scaring his little girl for life. Her whole life was planned out for her before she even knew what life was about. Plus, she was now scared of the dark because she knew things did go bump in the night. Parents are supposed to tell their kids there's nothing there or, at the very least, pretend to kill them. Or, in this case, actually kill them.
John repeated Samuel's mistake.
Regardless of the reason, Mary would not have . She wanted her kids to have a life. The one Mary tried to have. When Mary gets resurrected in the later seasons of , she is proud of her boys, but she would have liked them to have a different life because once someone is in it, it's hard to get out.
