In the article Storms gives seven reasons:
In the final analysis, virtually everything about healing remains a mystery. I don't mind saying that I'm weary of those who claim to reduce healing to a formula or a manageable cause and effect phenomenon in which we can know with certainty why some are healed and why others are not. I've labored in this meditation to avoid falling into that trap. That said, I would like to suggest that the reason why many are not healed may possibly be answered in any one of seven ways.
1. A lack of faith. (see the article for his explanations)
2. The presence of sin.
3. Some don't want to be healed.
4. They don't ask.
5. Demonic oppression has not been dealt with.
6. The mystery of God's providence.
7. Often times there are dimensions of spiritual growth and moral development and increase in the knowledge of God in us that he desires MORE than our physical health, experiences that in his wisdom God has determined can only be attained by means or in the midst of or in response to less than perfect physical health. In other words, healing the sick is a good thing (and we should never cease to pray for it), but often there is a better thing that can only be attained by means of physical weakness. More important to God than our physical health is our spiritual holiness.