Board Thread:Questions and Answers/@comment-10797017-20130624101208/@comment-8986886-20130710214603

The Reishi concentration in the atmosphere at the Soul King's palace seems to be abnormally high and combined with Ichigo's growth rate might allow him to learn Kidō much faster than the average Shinigami; especially with the Zero Squad as teachers.

Hadō #63 Raikōhō is supposed to be a trademark of the Shiba Clan, so it would make sense if Ichigo eventually learns this technique from either Kukaku or Isshin to continue the family tradition.