Quite simply because the rapture is a reward. Not all will get that reward just as with other rewards not all people will get them. You can be saved and not be raptured they are not the same nor are they intertwined(meaning 1 can't exist with out the other). Those who are not raptured will die in the tribulation. The weding isn't till the END of the tribulation by then all believers will be there. The bride will be whole. The main points here are : A) the rapture is not equal to salvation and B) the rapture is a reward.
 
Paul