Because the transporter works on power of plot, for some reason they couldn't just recreate Tuvix after getting Tuvok and Neelix back. But they can totally do that sort of thing if another episode requires it.
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Guys, calm down. It's just the trolley problem and she decided to pull the lever to save 2 people. If you disagree then you just have a different answer to the problem.
Nooooo! A made up character for a TV show made a moral blunder! We can't afford to support a made up character.
Seriously though I agree with above, stop getting worked up about this.
Okay, hear me out, if they could replicate the incident, did she actually kill him? Then also were Tuvok and Nelix effectively dead before recreating them?