I think it was chosen primarily because of the fact that it was easy to devide the creation "groups" into six; man, beast, air, etc. Another big reason was to establish a sabbath day. A week works out well for the plan.
And prior to creation (assuming that's how it happened), by what standard did god judge days? There were no cosmos, no sunrises, nothing to even suggest that time existed (nor is there nothing to suggest it actually exists now
)