Here's why:

* Primary structure refers to the linear sequence of amino acids in a polypeptide chain. This sequence is held together by peptide bonds, which are covalent bonds formed between the carboxyl group of one amino acid and the amino group of the next.

* Secondary structure (alpha-helices and beta-sheets) is stabilized by hydrogen bonds between backbone atoms.

* Tertiary structure (the three-dimensional shape of a single polypeptide chain) is stabilized by various interactions, including hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges (which are covalent).

* Quaternary structure (the arrangement of multiple polypeptide chains in a protein complex) is also stabilized by a combination of these interactions, including disulfide bridges.

While disulfide bridges can contribute to stabilizing tertiary and quaternary structures, they are primarily associated with the primary structure because they are covalent bonds directly linking amino acids within the polypeptide chain.