because the carboxyl group is deactivating and the catalyst aluminium chloride (Lewis acid) gets bonded to the carboxyl group.
Detailed Answer :
Because –COOH group present in aromatic carboxylic acids is an electron withdrawing group causing deactivation of benzene ring. This results in the bonding of anhydrous AlCl3 with carboxyl group. Hence, electrophillic substitution i.e., Friedel-Crafts reaction does not occur in aromatic carboxylic acids.
