Benzene diazonium ion ($[\text{C}_6\text{H}_5\text{N}_2]^+$) is a relatively weak electrophile and can only attack strongly activated aromatic rings containing electron-rich groups such as $-\text{OH}$ (phenol) or $-\text{NH}_2$ (aniline, o-toluidine). Nitrobenzene contains a strongly deactivating $-\text{NO}_2$ group which depletes electron density from the ring, completely preventing azo coupling.