Why is the effect of ring activators more dominant than ring deactivators? In 3-nitroaniline, if it were subjected to an electrophilic substitution reaction, the electrophile will most likely attack at (b) or (d) as mentioned in my textbook. 

To me, it seems that the only role of the nitro group is that the electrophile will not attack at (a) due to steric hindrance. So my question is, why is the -M effect of the nitro group ($\ce{-NO2}$) discarded and only the +M effect of $\ce{-NH2}$ plays a role? Nitro group is meta directing, so why wouldn't the product form at (c) as well? It is as if the nitro group doesn't even exist. In better words, why is the effect of the ring activator, $\ce{-NH2}$, more dominant than the effect of the ring deactivator, $\ce{-NO2}$?
$\ce{-NH2}$ activates the ring due to its +M effect, increasing electron density in the ring at (b) and (d). On the other hand, $\ce{-NO2^{-}}$ causes de-activation because of its -M and -I effect and the electron density would be higher at (c) relatively. 

Now since in activated rings, the rate of electrophilic aromatic substitution is higher than those in deactivated rings, the effect of $\ce{-NH2}$ as ortho-para director is seen more during product formation and not of $\ce{NO2^{-}}$ majorly. The electrophile will attack at (c), but since its rate will be quite slow, it would not form the major product.