Why has Allah forbidden the pilgrims to wear sewn garments, and what is the wisdom behind that?
Allah (Exalted be He) has ordained Hajj once in the lifetime for those who are accountable and are able and have the means to perform it. He has made it one of the pillars of Islam and Ma`lum minad-din biddarurah (well-established and known Islamic principles). Therefore, a Muslim has to do what Allah (Exalted be He) has ordained upon him to please Him and obey His commands; hoping for His reward and fearing His punishment. We must believe that Allah (Exalted be He) is Wise in regard to His legislation and in all that He does and that He is Merciful with His slaves. Therefore, He would not legislate anything for them but what is in their best interests and which will be of great benefit to them in this world and in the Hereafter. It is for our Lord, the Sovereign, the All-Wise, to legislate laws, and it is for His servants to obey and submit.
There is a lot of wisdom behind the command of avoiding sewn garments during Hajj and `Umrah. For example, it reminds us of how people will be on the Day of Recompense, for they will be resurrected barefooted and naked, then they will be clothed. Reminding ourselves of how things will be on the Day of Recompense teaches us an important lesson. It also makes us humble and feel that we are obliged to submit and purify ourselves of arrogance. It reminds us of the principles of rapprochement, equality, asceticism, and of keeping away from luxurious lifestyles which is condemned. It makes us feel empathy for the poor and needy and so on. There are other objectives behind performing Hajj in the manner prescribed (Part No. 11; Page No. 180) by Allah and explained by His Messenger (peace be upon him).May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.