Well I don't think it is unreasonable to assume that earth is one of the central worlds of trader space. I think it's not owned as such by any part of the TEC, as the frontier worlds are, but is rather property of all of humanity.

Being in the very center, it wouldn't be in major danger of Vasari or Advent attack....yet. The Vasari will be content if they can begin their next phase of the Exodus. The Advent, however, would love to wipe out Earth.

I always just considered it one of the inner core worlds. So at the moment the war is raging on the outer to middle colonies so you won't see it. Too tell you the truth at first I was disappointed that Earth wasn't a planet. I wanted to nuke, glass, and kinetic bolt it!

Another possible theory is that Earth has been destroyed or run out of resources to be of use. Sins doesn't have a starting date, but you know it has been a long time because the Trade Order had been around for 1,000 years and then changed to TEC when the Vasari attacked (as mentioned in the manual). And at that time humans were already settling other worlds. We also don't know when humans invented or discovered the technology to travel to other solar systems. It is a possibility that in that time Earth could have been hit by a large asteroid or all the resources were used up and instead of shipping supplies from other worlds to Earth, people just left.

You could come up with many different theories for this.  

I don't really care to know. I've played lots of games, watched TV, and watched many movies. Much of the selection that are space based seem have Earth as some place important merely because its... well... Earth. Having Earth be saved... again, or be some place important for the n-th time will not do it for me.

I'll be quite happy if the devs said that humanity has simply forgot where their place of origin was...