Siddhartha becomes disillusioned with the ablutions as well as other religious practices because he has learned them and practiced them faithfully -- just as his father and others have -- and he feels no closer to achieving enlightenment; likewise, neither his father nor any other elders have reached nirvana in this way. Since the point of the rituals and teachings is to help one to become enlightened, and no one in the community has ever reached enlightenment by faithfully repeating and learning them, it begins to seem to Siddhartha that the ablutions and other rituals are not actually the correct path. The narrator says, "He had started to suspect that his venerable father and his other teachers, that the wise Brahmans had already revealed to him the most and best of their wisdom, that they had already filled his expecting vessel with their richness, and the vessel was not full, the spirit was not content, the soul was not calm, the heart was not satisfied." He feels that he has nothing else to learn from his teachers because they've taught him all they can and he remains unenlightened. This is the cause of his disillusionment.
Siddhartha becomes restless and begins to question his father's beliefs. He feels that he has learned all he can from talking to the Brahmans and they can not give him the peace he seeks. As Hesse writes in the first chapter: "The ablutions were good, but they were water, they did not wash off the sin, they did not heal the spirit's thirst, they did not relieve the fear in his heart."
With all this stirring inside of him Siddhartha decides to go on a quest to find what is missing within himself.