The character was a loser.
7 years as an ensign, a dogsbody.
He did plenty to merit promotion but obviously never had the guts to stand up and ASK for what he deserved.
You see people like this in real life all the time sadly, they do a really good job but stay on the same salary or in the same position and complain that they are passed over for promotion constantly, and the reason is they are too scared to ASK for it, and expect to be offered it on a silver platter.
Kim should have marched into Janeways office and slammed down a bunch of mission reports and a bunch of comparative personnel files of Lieutenants who have done a lot less in their careers than he has.
Best case scenario Janeway promotes him, worst case scenario she doesn't.
She can't "fire" him, and she wouldn't punish him for asking for what he in all honesty had earned, so there's nothing to lose.
There's a reason it's called "climbing the ladder" it's a climb, you don't get a free ride up it, don't ask = don't get 9 times out of 10
But he wasn't that kinda guy. Same reason he never had a girlfriend in that 7 years
he was "mr nice guy"