Why was his claim to the English throne better the William's?

His claim to the English throne was considered stronger than William's because he was a direct descendant of King Æthelred the Unready, through his mother, which provided him with a legitimate lineage to the English crown. Additionally, he had been promised the throne by Edward the Confessor, who was believed to have named him as his successor. This prior connection and promise gave him a more solid basis for his claim compared to William, who was a distant cousin and relied on military conquest rather than hereditary rights.