the official take (wrt production of the show), i think, was that 'somebody has to be the ensign', and it's not like they were gonna get an influx of new academy graduates out in the delta quadrant.
He became a test pilot after VOY, and LD only takes place a few years after that. It’s perfectly possible that there isn’t a lot of room for advancement in that particular career field.
I don't really care for screen rant. TLDR: He didn't want a promotion because he wanted to keep flying and he's immature.
I can sort of see wanting to keep flying, but you'll never convince me none of th senior officers he lived with for 7 years wouldn't have mentored and pushed him to be a great officer.