The 3 week wait is, primarily, looking ahead to combat. It's so that you can't die, respawn at your faction's HQ, get in a new ship, and dive straight into the fight. There are additional factors, but this is the oldest explanation I've heard.
Death is a penalty, in this game, and the 3 week wait is designed to reflect that, making it undesirable and so that a new character is more of a fresh start.
Why 3, and not 2 or 4? *shrugs* It's a slightly arbitrary number, but presumably 3 is thought to be not too long so people quit, but long enough to fulfill the mentioned requirements.