But, Voyager could have easily brought on supporting crewmembers for longer. Half the recurring minor characters they unceremoniously just dropped. Carey stopped appearing except for one random appearance in Fury just to tell us when we were back in time again and one other appearance just to die, Hogan was eaten by a giant lizard-snake, that other guy got his face ripped off. They could have had more minor characters who played a larger role in the story.
The actors very well may not have been available to reprise their roles that often. For example: many of the survivors of the Equinox were doing other shows and guess spots and very well could have been the reason why the writers couldn't revisit those characters.
Second is budget.
If they felt they didn't have the budget to keep Jen Lien and add Jeri Ryan, they might not have had the budget to pay those actors their salary. I can assume, the young newer actors like Manu & the other Borg kids work for much cheaper just for exposure.