why 'it' is called as it ?

More information is needed to answer.♥

Do you mean," why 'it' is called an it"?

Because it is a thing and not a person.

O.E. hit, neut. nom. & acc. of third pers. sing. pronoun, from P.Gmc. demonstrative base *khi- (cf. O.Fris. hit, Du. het, Goth. hita "it"), which is also the root of he. As gender faded in M.E., it took on the meaning "thing or animal spoken about before." The h- was lost due to being in an unemphasized position, as in modern speech the h- in "give it to him," "ask her," "is only heard in the careful speech of the partially educated" [Weekley]. It "the sex act" is from 1611; meaning "sex appeal (especially in a woman)" first attested 1904 in works of Rudyard Kipling, popularized 1927 as title of a book by Elinor Glyn, and by application of It Girl to silent-film star Clara Bow (1905-1965). In children's games, meaning "the one who must tag the others" is attested from 1842.

http://www.etymonline.com/index.php?term=it

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no idea, but I used to ask questions like that all the time.