I'm less familiar with Crusher leaving by my supposition was it was a temporary posting, possibly resulting in a temporary promotion to a medical captain while the permanent officer for heading up Starfleet Medical was found.Possibly TNG era people, even in Starfleet, are not as career minded as TOS era people or 21st century people.
I think that Crusher did get a big but temporary promotion in rank as "cheif of Starfleet Medical". That certainly sounds like a flag officer worthy position. No doubt it was a temporary or acting promotion dependant on her positinas chief of Starfleet Medical, and when she decided to return to the Enterprise and her son she automatically dropped her flag officer rank.