The main reason is that Nitro group is having more -I effect at the ortho position as compared to p-position. As -M effect is similar im both the cases, the deciding factor for more acidic character is high -I effect of NO2 at ortho-position as well as the ortho effect (Here probably the intra-molecular H-bonding which helps in further increase in acidic character). Due to this ortho effect even the electron releasing group attaced at ortho position increases the acidic character of the carboxylic acid(for eg. ortho-hydroxybenzoic acid is more acidic compared to p-hydroxybenzoic acid).
