Hemingway used Nick Adams as a character for much of his early writing life, and then left him behind.
In the Nick Adams Stories, a collection of Nick stories and pieces, some not published elsewhere, Hemingway gives some indication why he left Nick behind.
There is the beginning of the WWI novel called "Night Before Landing" where Nick is in transit on a troop ship to Europe. Nick proved an inadequate character for his WWI novel, and was tossed aside (of course, Frederick Henry would take the torch in A Farewell to Arms). The novel was a dead end.
The key to Hemingway's abandonment of Nick may lie in a piece called "On Writing," which is weak and disjointed, but provides clues to Hemingway's ultimate estrangement with Nick Adams. In this story, Nick is a writer, and he muses on writing. He says in one part:
"The only writing that was any good was what you made up, what you imagined. That made everything come true. Like when he wrote "My Old Man" he'd never seen a jockey killed and the next week Georges Parfement was killed at that very jump and that was the way it looked. None if it had happened... That was what the family could not understand. They thought it was experience."
Here, Hemingway cleverly gives Nick Adams an experience that Hemingway had, where Nick has written the story "My Old Man" where a jockey is killed from fall from a horse. The fall didn't happen, but later, Nick saw a real fall and a real jockey's death at the same track and it was exactly like his writing, which he imagined. Nick's family cannot distinguish that he makes things up... they think that he is writing strictly from experience, and that hurts them since Nick Adams is embedded so much in family life.
Of course, it was Hemingway who saw the jockey really fall after writing "My Old Man." Here he gives Nick the experience while claiming that writer's never (or should never) write about experience. You work out the logic . There is a fascinating circularity here, maybe even a joke, and a clue. Nick was too close to Hemingway, both for himself and for others. He needed to leave that character behind to get at the imagined world of pure writing.
All the late Nick Adams pieces in The Nick Adams stories contain this deflated quality. Hemingway had already written Nick out. He just hadn't fully realize it until he was certain he was dead and gone and had written too many stories with him.