The amide is a less
powerful activating group than the simple amino group, -NH2 since
resonance within the N-acetyl group of the amide (see below) competes with
delocalisation of the N lone pair into the ring.
Steric effects in the amide also
often lead to a decrease in the amount of the ortho-products.
In principle, the sequence
protect - substitute - deprotect is equivalent to being able to substitute
the aniline directly.