It turns out Jetrel has been trying for the last 15 years to find a way to bring back the dead. Neelix asks the obvious question: why didn’t he say this to begin with. His answer is that even Janeway, and accomplished scientist, doubts his theories. That doesn’t really answer the question, and it also doesn’t answer the question of why he was so cold to Neelix earlier in the episode. I know the answer: at that point the audience was supposed to be unsure about Jetrel’s motives, so they made him a jerk.