Why is the Indian caste system considered to be rigid and not fluid

Because it is determined by birth.

Explanation:

India caste system is a social stratification system in which ones future or destiny is determined by his birth. You will be rich if you are born in rich homes and same applies to poverty. It is one of the world's oldest caste system which began in the 14th century.

i think the reason that they immagrated was to find freedom of religion