Here's why:

* Methylation is a common modification in rRNA: Methylation of adenine and guanine bases is a crucial modification in rRNA. These modifications play a role in rRNA structure, stability, and function.

* Other RNA types may have methylation, but not as prevalent: While other RNA types like mRNA and tRNA can have some methylation, it's not as widespread or essential as in rRNA.

Let me know if you have any other RNA-related questions!