While Harry Kim wasn't the best character, and Wang certainly wasn't the best actor, in skill or work ethic, whether or not a character gets promoted has nothing to do with development. And Harry had more than one focus episode. In fact, he had far too many for my tastes. But around here, character development seems to be largely subjective.
Yes, he had way too many episodes, but he was blank as far as characters come. One dimensional. Just because he displayed emotions or played the clarinet doesn't really give him any dimension. Having episodes also doesn't mean he had depth. In a few of them he did, but across the series he was lacked depth and development, even in some of those episodes he still lacked it. It's like he was written as the green baby of the crew and then he barely grew beyond that. Most of the characters evolve. He does but just barely and only in rare moments do we see it.