Because not everyone can be promoted on Voyager.
...Or because no one
It's not as if we ever really saw a promotion in that series, after all. Tom Paris got his Lieutenant pips apparently as part of the deal made with Starfleet back on Earth; he then got half a pip taken away for some reason, and then again half a pip taken away. And then he got half a pip back, which wasn't really promotion but reversal of punishment. Tuvok got half a pip taken off his collar for some reason, and then very publicly was given it back.
Perhaps Janeway simply felt that she did not have the right to give any field promotions to anybody? At least not regular Starfleeters, that is - but even the Maquis only ever got demotions after they had received their initial rank bars.
Out-universe, I doubt rank of a character was ever an issue a writer, a producer or even an actor would feel the need to think about. Except of course when the plot involved the rank changing. But demotion would have been as good publicity for all the parties involved as promotion, surely.