What is the rationale for forbidding form-fitting clothing for a person while in the state of Ihraam?
All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is impermissible for a person in a state of Ihraam (ritual consecration) to wear form-fitting clothing which is clothing that is tailored in a way that fits or forms the body-parts such as shirts, pants, and cloaks. It does not mean anything that has thread or is sewn or stitched, as some people mistakenly believe.
This is based on the Hadeeth of the Prophet (may Allah exalt his mention), where he said: "A person in a state of Ihraam should wear neither a shirt nor turban." [Al-Bukhari & Muslim]
The wisdom behind this, and Allah knows best, is that this style urges a person to maintain a state of Khushoo‘ (humbleness and submissiveness to Allah The Almighty). It also reminds one of the Hereafter because, in such a state, he relinquishes the luxury and apparent adornment of the worldly life.
Moreover, this dress, which is worn by all pilgrims, represents the unity of the Muslims and equality between them. Hence, there is no room for anyone to boast or mock the dress of another. If the matter was left for the people to choose, the rich and poor would be distinguished from one another as would the rulers and subjects.
Furthermore, the clothing of Ihraam inspires a pilgrim with the spirit of Ihraam, so he indulges in intensified supplication and Thikr (mentioning of Allah) and guards himself against all prohibitions and forbiddances.
Allah Knows best.