Niles spends over a year on Earth quietly investigating irregularities in SKAEL's behavior that no one else at the Initiative takes seriously enough to act on. When the mission needs someone who already understands what SKAEL might be becoming, he is the only qualified candidate.
His reassignment to Mars puts him in an unusual position: he has been reading the crew's transmissions for months before meeting any of them, and arrives already more familiar with their situation than they are with his.