An Hadith is referred to as da'if because

An Hadith is referred to as "da'if" because it has defects in its chain of transmission (isnad) and/or the text of the tradition (matn). This means that one or more of the transmitters in the chain of transmission may not be reliable or the text of the tradition may contain inconsistencies or contradictions. A hadith is considered da'if or weak because its authenticity is not guaranteed, and it may not be used as evidence in Islamic legal or theological discussions.