Here's why:

* Benzene's stability: Benzene is an aromatic compound with a highly stable structure due to its delocalized electrons. This stability makes it relatively unreactive.

* Bromine's electrophilic nature: Bromine is an electrophile, meaning it seeks electrons.

* The need for a catalyst: The reaction of benzene with bromine requires a catalyst, typically a Lewis acid like iron(III) bromide (FeBr3). This catalyst helps to generate a more reactive electrophile (bromonium ion) that can attack the electron-rich benzene ring.

In the absence of a catalyst and light, the reaction between benzene and bromine does not occur.

However, if exposed to light, the reaction can proceed via a free radical mechanism, but this reaction is much slower than the catalyzed reaction and produces a mixture of products, including bromobenzene and polybrominated benzenes.