Aryl halides are less reactive towards nucleophilic substitution reaction as compared to alkyl halides due to

Aryl halides are less reactive towards nucleophilic substitution reaction as compared to alkyl halides due to resonance stabilization.

Due to resonance, C−Cl bond acquires partial double bond character and becomes shorter and stronger and cannot be easily replaced by nucleophiles.

Other reasons for the low reactivity of aryl halides are

(1) Difference in the hybridization states of carbon atom in C−X bond. 

In alkyl halides, the carbon atom is sp³ hybridized whereas in aryl halides, it is sp²  hybridized and hence, more electronegative. Hence, C−X bond in aryl halides is more difficult to break.

(2) Polarity of C−X bond in aryl halides is lower than that in alkyl halides. Lower is the polarity, lower is the reactivity.