Axiom's (and the other cruise spacecrafts') function was to safeguard humanity for generations as it cruised through space until the EVE probes returned promising results from Earth.
According to the wiki page, the Axiom is located in the Kuiper belt.
That's too far away from Earth! Why didn't it just orbit Earth? Maybe there was too much debris in Earth's orbit. Well then, it could orbit Mars or something like that. Why go all the way to the Kuiper belt?
This way, the Axiom can more easily monitor Earth's status and whatnot.