As the n ame of author appears after the specific epithet.....we can also write it as Homo sapiens (Carl.)....but then why is it neccessary to write Linn. ?

The name of the author who first scientifically named and published a description of human being as Homo sapiens is Carolus Linneaus. As Linn. is used as standard abbreviation for the authors name in biology and not Carl., we write Homo sapiens. (Linn).