NO2 group shows its effect only at the ortho para position why ? 

In Nitrobenzene, the resonance structures have positive charges around the ring system:

Every functional group shows its maximum effect on the ortho and para positions only as these two positons are in the plane of the ring unlike the meta position.

As we know that NO₂ group is Electron withdrawing groups (EWG). It pulls the ring electrons towards itself. (See the Diagram).

Due to this pulling of electron; it decreases the electron density at ortho and para position of the ring (+ sign at o and p position). But, meta position remains unaffected during this movement of electron.