Why does God allow sickness?
One of the reasons our God may not make a sick person well is so the person does not lose his reward in the afterlife. This is similar to a father not giving all of the inheritance to his children at once so they don’t go on to waste it. We should always consider this and the afterlife as a continuum then we can understand that God may have plans to reward us in the afterlife.
From the book “From the Ascetic and Hesycastic Athonite Tradition” Translated by Anna Pipinos and Elias Pipinos.