The nitration of nitrobenzene at room temperature gives 1,3-dinitrobenzene, but not 1,3,5-trinitrobenzene, despite the use of excess reagents. Why is the reaction selective?

Question

I think that under ordinary circumstances, at room temperature, some trinitrobenzene is formed, but not very much. The addition of a -nitro to the ring makes is much less reactive, and when two have been added, the ring becomes virtually unreactive.