why is anticlockwise moment taken positive and clockwise negative
It follows from general convention which is handed down to us from generations and is universally accepted. Anohter reason could be that...
A charge particle be it an electron or a proton would move along a circular path in uniform magnetic field. However the direction of motion would depend upon the magnitude of charge as positive charge would move in an anti-clockwise orbit while a negative charge would move in a clockwise orbit.