There is an episode of TNG where Riker declines leaving to take the captain's seat on another ship and remains on the Enterprise under Picard. It may be that, frequently, promotions in an organization like Star Fleet are given with a transfer of station.

Rank impacts chain of command, and is generally reflective of seniority. But much in the same way that your uncle will always be your uncle, Voyager was a several years long command structure held in a family-like stasis. They could have doled out promotions on a regular basis, but that would just lead to a whole bunch of commanders under Captain Janeway, and then the command structure would rely exclusively on seniority.

I'm sure everyone who made it home got a promotion as soon as they accepted their next assignment.