In the Voyager episode Mortal Coil, Neelix is brought back to life after being dead for eighteen hours using Seven's nano probes.
What's the in-universe explanation for this not being used to revive anyone else in the series?
While I can't omit the possiblity of it being forgotten about by the writers, there is also this: http://en.memory-alpha.org/wiki/Starfleet_casualties_(24th_century)#USS_Voyager_.28NCC-74656.29
Mortal Coil, where the Borg revival technique was first used, aired after almost all the confirmed deaths. The only exceptions are:
Due to this massively smaller death toll after Mortal Coil, I would argue that they were using it, just not onscreen. As for these 6 people? Yeah, the writers probably forgot about it.
Possible in-universe theories:
Ahni Jetal's death was caused by an attack from some alien weapon.
It may have disrupted the nanoprobes. In this part of space (this
part of the series), there were quite a few species actively building
weapons to combat the Borg.
Lyndsay Ballard's description of her death was a little inconsistent. The stardate places it after Mortal Coil, but according to Memory Alpha, her other descriptions all put it before Mortal Coil.
Death due to nucleonic radiation & 3 deaths due to a lifeform from another dimension. The circumstances of their deaths may have been enough to disrupt the nanoprobes so that revival would not work.
EDIT - I'm watching Latent Image now, and it turns out Ahni Jetal died 18 months before that episode - the Doctor even states that it was before Seven came aboard. So we're down to 5 deaths after Mortal Coil.