Because the prayer in which the worshipper does not perform the bowing and prostration completely is not sufficient for him.

Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "That prayer is not sufficient in which a person does not straighten and level his back in bowing (ruku) and prostration (sujud)."

Hazrat Ali bin Shiban, who was a member of the (delegation that came to serve the Prophet), says that I offered prayer behind the Messenger of Allah (peace be upon him). The Prophet (peace be upon him) glanced sideways and saw a man who was not straightening his back during bowing (ruku) and prostration (sajdah). Then, when the Prophet (peace be upon him) completed the prayer, he said: "O assembly of Muslims, indeed the prayer of the one who does not straighten his back during bowing and prostration is not valid." This is the hadith of Ahmad bin Al-Muqaddam.