I don't think it's fair to say that a species that only lives 9 years can't evolve past primates. They would just need to develop social structures that are good at transferring knowledge. We can't assume that just because humans can't absorb a lot of knowledge in nine years that it's impossible with a specialized brain. We don't know under what conditions other species evolve.
Humans' fluid intelligence peaks at 13 yet we consider our prime to be the 20s. Why can't Ocampa fluid intelligence peak at 2? They wouldn't have the crystallized intelligence, but again, maybe their social structures compensated for that, and maybe they had shepherd races like the Caretaker to protect them.
They simply won't have the time to create anything. Not to mention that the Ocampa didn't show any signs of fast creating or building. They did seem rather placid.
The Caretaker didn't seem to involve himself in too much of their lives either. He just let them live their underground life. He didn't even wipe out the Kazon from the surface, despite knowing that they were a threat to the Ocampa.
She would leave for the same reasons she left Ocampa with strangers, to see what she couldn't on Voyager from a different perspective.
I never assumed Earth was going to be her final destination seeing that twice in the past she was eager to leave Voyager to and take her exploration of herself and space in a new direction.
Everybody has family but most as they grow up, move out and away from their family to explore and start a life on their own. If Kes could say good-bye to her family/people and life on Ocampa without batting an eye, you think she wouldn't do the same to a ship full of people she wasn't related too? Again, she already tried twice. If she's that eager to leave, then eventually Janeway's "Please, stay" speech wasn't going to work.
She left Ocampa because she found the whole society boring and backwards. She stayed on Voyager because she liked it there. She had friends and could travel, explore and learn which was what she wanted. Such things happen in life.
As for the writers and their "problems " with writing for characters like Kim, Chakotay etc. They didn't kick out those characters like they did with Kes, that's the difference. If the solution was to kick out the characters they suddenly became incapable to write for, why didn't they dump everyone except Seven? Or why didn't they kick out themselves since they were incapable to do a good job? That would have been the best solution.