Here's why:

* Transcription is the process of copying genetic information from DNA into RNA.

* mRNA carries the genetic code from the DNA in the nucleus to the ribosomes in the cytoplasm, where protein synthesis occurs.

While other types of RNA exist (like tRNA and rRNA), they play different roles in protein synthesis:

* tRNA (transfer RNA) carries amino acids to the ribosomes.

* rRNA (ribosomal RNA) is a component of ribosomes, the cellular machinery responsible for protein synthesis.