Here's why:

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

* Short lifespan: Once the protein is synthesized, the mRNA molecule is no longer needed and is rapidly degraded. This ensures that the cell doesn't waste resources producing unnecessary proteins.

* Regulation: The short lifespan of mRNA allows for quick and efficient regulation of protein synthesis. Cells can quickly adjust their protein production based on changing environmental conditions.

Other types of RNA, like rRNA and tRNA, are generally more stable and long-lived because they play essential roles in protein synthesis and are needed continuously.