When using a relative pronoun to introduce a cause of something, one can use either why or that.

This is the reason that both the web interface and the booking board show only seven days of reservations

[T]here must be an underlying ideological, ritual, sociological, or political, if not operative, reason why the dead and/or those who have buried them would choose to mark a person in death in a particular way (Costin 1996:119).