Next issue, #58, we would expect to see the start of a short backup serial regarding Jean Grey's origin and how she was recruited by Xavier to be part of the X-Men. This does not happen. Instead, the main story goes back to being 20 pages, for the first time since #37. This is what's really telling. I've seen it claimed that this happened because Jean didn't need this treatment - she was already shown arriving at the mansion back in #1. This strikes me rather as clutching at straws. Yes, X-Men #1 has her arriving at the school not knowing anything about it (having received a message from the Professor somehow, maybe he sent an owl or something). But this is Xavier here we're talking about, who is a little trigger-happy with the mindwipes and, in that same issue, claimed to have lost the use of his legs in a childhood accident.
And clearly, there is an interesting backstory for Jean Grey that could be told. We know this because Chris Claremont would tell it twelve years later, in Bizarre Adventures #27. We learn of Annie, Jean's childhood friend, who died in a car accident when they were young, and Jean's subsequent shutdown when a part of her telepathically sensed Annie's dying moments. It is quite dark stuff for comics - a deeply lasting childhood trauma versus Scott's romanticised orphanhood, and Hank and Bobby's painful but ultimately consequence-free outings. It certainly could not have been told in X-Men in 1969.
The story has been revisited from time to time. 2008's X-Men Origins: Jean Grey (Sean McKeever/Mike Mayhew) and the 2011 Marvel Girl one-shot (Joshua Hale Fialkov/Nuno Plati) both look at Annie's death in the context of Jean coming to terms with it early in the O5 period. Marvel Girl weaves into this a rather wonderful plotline about Jean discovering a supervillain who is trapping people in their pasts, only for her to realise that's just what happens to some people... If the 1960s had been a little less sexist, and we'd got our Jean Grey origin backups, we'd be denied a place for a powerful story of loss.