And what about if Harry's fate - not being promoted - was linked to the way his performer behaved on behind the scenes?! (would we really like to promote a guy who mess with the production and his acting work? The answer is no!)
-> Chakotay was promoted too, like B'Elena and Tom Paris but in Janeway's mind, all that was temporary, the time of the journey. Of course, she could have done the same with Harry but well, she chose to treat him as more than a simple Enseign in words and facts. This is not insignificant!