An Hadith is referred to as da'if because

An Hadith is referred to as da'if (weak) because it has defects in its isnad and matn. Isnad refers to the chain of transmission of the Hadith, and matn refers to the actual content of the Hadith. If the chain of transmission is weak or unreliable, or if the content of the Hadith is inconsistent with other reliable sources, the Hadith is considered da'if. This does not necessarily mean that the Hadith is fabricated, but rather that its authenticity is uncertain, and it should not be used as a basis for religious teachings or practices.