the actuall cause is the normal climatic conditions there, bcuz there r well balanced seasons not tooo chill(although summer is on the top) but normally their medium color toned skin always remain normal(mostly).the water is slat free,n in west the refined n cleaned water from salts (gud to drink) is very bad for hair n skin,,it dries out the skin n gradually makes it dull,,,n in west the mostly food is like not organic,also here food habbits include like sweets eating chocolates ect,,,n fry food is very commen...all these factors make the asian's skin better although there is more dust,more hot.
I have noticed that that water in Pakistan bleaches my skin. Instead coming back tanned, I always come back lighter.