According to a textbook by K. L. Chugh:
Sulfonation [of aniline] is a reversible reaction. The p-isomer, being more stable, does not get desulfonated easily. Hence, p-sulfanilic acid is the major product.
I am curious as to why the ortho isomer (2-aminobenzenesulfonic acid) is not more thermodynamically stable. There is a possibility of intramolecular hydrogen bonding (in a six-membered ring):
Why is the para isomer (4-aminobenzenesulfonic acid) favoured?