Question

Conventionally in an electric circuit the direction of electric current is taken as opposite to the direction of the flow of electrons which are negative charges why

Answer 1

To provide a definition of current independent of the type of charge carriers, conventional current is defined as moving in the same direction as the positive charge flow. So, in metals where the charge carriers are negative, conventional current is in the opposite direction. 
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Answer 2

By convention we take direction of flow of positive charges as positive and those of negative charges as negative. Current is defined as rate of flow of charge. here charge is generally of electrons, which is negative, hence the direction of current is opposite to that of direction of motion of electrons.