Q.Why are haloarenes less reactive than haloalkanes towards nucleophilic substitution reaction?

Resonance in haloarenes strengthens/shortens the C–X bond (partial double-bond character) and makes the ring electron-rich (repelling incoming nucleophiles); additionally the sp$^2$-hybridised C–X carbon and the instability of a phenyl cation/steric hindrance to backside attack block both S$_N$1 and S$_N$2 pathways.

Several factors make haloarenes far less reactive than haloalkanes towards nucleophilic substitution:

1. Resonance: The halogen's lone pair conjugates with the aromatic π system, so the C–X bond acquires partial double-bond character. This makes the C–X bond shorter and stronger than a normal C–X single bond (as in haloalkanes), so it is harder to break.
2. Difference in hybridisation of carbon: The C–X carbon in a haloarene is sp$^2$ hybridised (vs sp$^3$ in a haloalkane); sp$^2$ carbon is more electronegative and holds the bonding electron pair (and the halogen) more tightly, again strengthening the C–X bond.
3. Instability of the phenyl cation: An S$_N$1-type mechanism would require formation of a phenyl cation, which is highly unstable (the empty orbital would be in the plane of the ring, not conjugated with the π system), so this pathway is disfavoured. …