Why is voluntary night prayer an honor for the believer, even though this virtue is not mentioned regarding the obligatory prayers?

Muslims share in the performance of the obligatory prayers, but distinction in ranks and the attainment of honor are achieved through increasing supererogatory acts and good deeds. Qiyam al-layl (night vigil prayer) is an honor for the believer, for the Prophet, peace and blessings be upon him, said: "Know that you will not prostrate to Allah a single prostration except that Allah will raise you a degree by it and remove a sin from you by it," and he also said: "The honor of the believer is in Qiyam al-layl, and his dignity is in his independence from people."