why does wilbur get sold to the zuckermans?

At five weeks old Mr Arable decides that Wilbur is ready to sell. Fern is distraught but her father is firm. Fern's parents decide to compromise and sell Wilbur to the Zuckerman's' just down the road so that Fern can visit. Because he is a runt, Wilbur is sold for only six dollars and goes to live in Zuckerman's' barn.