User blog comment:BROTHER BAN-KAI/514. Born in the Darkness/@comment-5028409-20121030151858/@comment-4530178-20121031103721

You can only gain fullbring abilities if one of your parents were attacked by a hollow. In other words, either Isshin or Masaki were attacked by a hollow before Ichigo's birth, leading to Ichigo gaining fullbring abilities.