Here's why:

* Function: mRNA's role is to carry genetic information from DNA in the nucleus to the ribosomes in the cytoplasm, where proteins are synthesized.

* Structure: To perform this function, mRNA needs to be able to move through the cell and interact with ribosomes. A single-stranded structure allows for flexibility and easier movement.

* Comparison to DNA: DNA, on the other hand, is a double-stranded molecule, which provides greater stability but makes it less suitable for transport within the cell.

While mRNA is single-stranded, it does fold into a complex three-dimensional structure due to base pairing within the molecule. This structure is important for its function and recognition by ribosomes.