In terfenadine, the R group is a methyl group. It is electron donating and ortho/para directing in nature. Since the alkyl group in terfenadine is branched and bulky, so due to stearic hindrance the chances of formation of ortho substituted isomer are less likely. The main product obtained is para substituted isomer. Thus, synthesis is suitable for large scale purposes.
Fexofenadine can be synthesized by the same route used for terfenadine but first step gives unwanted meta isomer along with para product. The problem in the first step is the R group on the substituted aromatic ring.
It is electron withdrawing (carboxylic group) in nature and weakens the directing ability of alkyl side chain in fexofenadine. So the Friedal Crafts acylation results in meta and para products. These products have to be separated by chromatography which is a cumbersome and inconvenient.