Question

Why are aryl halides less reactive than alkyl halides towards Nucleophilic Substitution reactions?

Answer 1

The reasons for less reactive nature of aryl halides are: 
1- Resonance effect as in aryl halides there is a conjugation of electron pairs of halogen atom with pi electrons of the benzene ring. This delocalises the electrons attached on C-X bond which becomes a double bond in return and hence its cleavage is difficult.
2- No electron stabalization group is present that may conjugate with halide group.