Ok, theory----
The Losties, Others and the Freighties are 'at war' over the island. The helicopter that can get off the island to the boat only holds so many people --- get the important ones off....
Sun- who is preggers
Aaron - just a baby and his mother is dead (I think via the blast to the house and the 'headache' she couldn't sake this episode)
Kate- because she is hurt int the battle somehow and needs med attn asap
Jack- to save Kate's life - the medical bay on the boat
Sayid - who knows how to drive a boat/fix all the communications equipment
Hurley - don't have a good theory on him yet (maybe he knows how to fly a chopper?)

All the others sacrifice staying behind so those 6 can leave the island safely.
Hence, Jack's statement about Sawyer choosing to stay, and that he saved her.