Question

why electron withdrawing groups are meta directive not ortho para

Answer 1

Because when a electron withdrawing group is added, it creates +ve charge on benzene ring on meta directrive, so it e- withdrawing

Answer 2

Meta directing group are usually those groups which have -M effect (withdrawal of electron via mesomeric effect). Therefore if you draw the Resonance structure of such compounds you will find positive charge coming on the ortho and Para position only, since Meta position doesn't get positive charge it is safe to say that a nucleophile would attack at that position (since electron deficiency is less at meta). Note I've used the term electron deficiency because the electron density at Ortho and Para(And not Meta) is decreased due to - M effect