Here's why:

* Primary Structure: This refers to the sequence of nucleotides (A, T, C, G) in a DNA strand. It's like the alphabet of the genetic code.

* Secondary Structure: This is the double helix, the iconic twisted ladder shape formed by two complementary DNA strands held together by hydrogen bonds. Think of it as a word formed from the letters of the primary structure.

* Tertiary Structure: This usually describes the three-dimensional arrangement of a protein molecule, which is determined by interactions between different parts of the protein chain. There is no similar "folding" or complex arrangement in DNA like there is in proteins.

DNA's structure is primarily focused on the double helix (secondary structure), which is essential for its function in storing and transmitting genetic information. While there can be some variations in the double helix based on factors like supercoiling and interactions with proteins, these are not considered a separate "tertiary structure" in the same way as proteins.