Because John’s father, Reese was sent back in time (in Terminator 1) to save Sarah Conner (john’s mother), and ends up falling in love with Sarah. While on the run from the big bad terminator sent out to kill Sarah, reese and sarah do the hippidiedoo and John is conceived. So if Reese dies, there is no John, and if there is no John, there is no resistance. Talk about a paradox!