Here's why:

* tRNA (transfer RNA) is responsible for carrying amino acids to the ribosome during protein synthesis. Its unique three-dimensional structure, including the cloverleaf shape, is crucial for its function.

* mRNA (messenger RNA) carries the genetic code from DNA to the ribosome, where it's used to create proteins. It doesn't have a cloverleaf structure.

* rRNA (ribosomal RNA) is a key component of ribosomes, the cellular machinery that synthesizes proteins. It also doesn't have a cloverleaf structure.

Therefore, the cloverleaf structure is specific to tRNA.