I think that with all of Mary's family being dead/hidden, that John wouldn't have had any points of contact to find out about her, or even have a thought to investigate that angle. And even then, if he did, and maybe he did, all he'd come up with is no living persons, and either investigating dozens of other deaths on the offchance there was some meaning, or stay hyperfocused on Mary.
Although I do wonder that he ran into no one who knew of Mary Campbell in the hunting community, but he probably didn't share much, and seemed to be prickly and on bad terms with them anyway. Can't see him alternately pissing off hunters and spouting, "My wife Mary Winchester, whose maiden name is Campbell, was killed by a demon". More like, "My wife was killed by a demon".
New annoying fic quirk: "and he certainly wasn't thinking about that/him/it/[gay love], nope, not at all." The fact that it pops up so frequently, and the fact that the pov/narrator's voice is all wrong for that statement.