Here's why:

* The nucleus contains the DNA: DNA is the blueprint for all RNA molecules.

* Transcription happens in the nucleus: The process of transcribing DNA into RNA (messenger RNA, ribosomal RNA, transfer RNA, etc.) occurs within the nucleus.

* RNA processing occurs in the nucleus: After transcription, RNA molecules undergo processing steps like capping, splicing, and polyadenylation within the nucleus before being exported to the cytoplasm.

While some RNA processing steps may occur in other cellular compartments, the initial transcription and most of the processing of RNA takes place in the nucleus.