The Lewis structure for NCO-
If you count electrons and determine the formal charge on each atom, you find that in structure #1, the negative charge is on the oxygen.  Do the same exercise for structure #2 and you find that the negative charge is on nitrogen.  Since oxygen is more electronegative then nitrogen, the negative charge is more stable when its on the oxygen atom.  Therefore, structure #1 is more stable (preferred) than structure #2.

Structures #3-#5 are even less stable.  They either violate the octet rule (#4, #5) and / or have charges (sometimes multiple) on more than one atom (#3-#5).