bcos he did something bad in his previous life. Thro casteism and reincarnation BS, the brown aryans ruled the dark brown lo castes. Of course as time went by,it developed into something elaborate and thus form the cultural pillar hindus can't do without. Just like arabs can't do without sunni-islam. 

Occupation clans existed in many old cultures, euro,japanese and chinese included but there was no religion to enforce such division and they disappeared as social mobility increased. The average chinese are hardly aware of the origin of the chinese name 張