Why did Wilbur have to leave fern?

Wilbur had to leave Fern because she could no longer keep him as her pet; her father insisted that she sell him, as he was getting too large and needed more space. Fern was heartbroken but understood that Wilbur needed a proper home, leading her to sell him to the Zuckermans, where he would be raised as a farm pig. This transition marked the beginning of Wilbur's adventures and friendships at the Zuckerman farm.