Maybe they were born into a legalistic family. Maybe they were prompted by their insecurity to search out some rigidly structured system of religion. At any rate, we have found by experience that legalism is a difficult disease to root out.
People refuse to budge from legalistic systems because of fear.
Legalists usually teach a rigid system of rewards and punishments based upon an external performance of rites and the unfailing observance of the specifically stated demands of an inviolable moral code. When a convert enters the system, he is immediately apprized of what he can and cannot do on pain of God's withdrawal and the threat of hell fire. God is pictured as a petulant despot whose feelings are so sensitive that he takes offense at the slightest deviation from his expectations. Great wrath, revenge, and eternal punishment of indescribable horror are his weapons, according to the legalist. So fear becomes the goad that drives the convert deeper and deeper into a system that offers only neurotic rewards.