Because the gunman was too young to buy a gun; thus he took one from a “secure storage place” where his family kept the gun.
According to Oregon Live, the gunman’s name was Jared Michael Padgett. He was fifteen years old.
Senator Joe Manchin’s (D-WV) gun control bill would have extended retail background checks to even private purchases so that every gun purchase included a background check. He offered this bill in the wake of the heinous crime at Sandy Hook Elementary and admitted that it would not have stopped the crime at Sandy Hook.