Question
Why is it permissible to make worldly supplications in voluntary prayers, but not in obligatory prayers?
Answer
I say, and with God's success: It has been reported from the Prophet, peace be upon him, that he would limit the obligatory prayer to glorification (tasbih), and this is acted upon in it. There are also reports in individual hadiths regarding supplication, so it is applied to the voluntary prayers based on the evidence, and God knows best.