The reason why the pilgrim in ihram takes off his ordinary clothes
Question

                What is the reason why the pilgrim in ihram does not wear tailored garments during Hajj and ‘Umrah?.
Praise be to Allah.
      The reason why he takes off his ordinary clothes is so that 
      he will be completely humbled before his Lord, may He be glorified and 
      exalted, both outwardly and inwardly, because wearing the rida’ (upper 
      garment) and izaar (lower garment) is a kind of humbling oneself. You will 
      see the richest of people who could wear the finest of garments looking 
      like the poorest of people, because he is humbled completely. It is also a 
      show of unity among the Muslims, for they become one ummah even in dress; 
      they circumambulate one building, they stay overnight in one place, they 
      stone the Jamaraat in one place. 

    A third benefit is that the individual is reminded that when he departs this world, he will only depart in garments like these. He will never depart in the finest clothing; rather he will depart in a shroud. And Allah is the One Whose help we seek. End quote.