Here's why:

* Transcription: This is the process where a gene's DNA sequence is copied into a messenger RNA (mRNA) molecule. This happens inside the nucleus, where the DNA resides. The mRNA then carries this genetic information out of the nucleus to the ribosomes.

* Translation: This is the process where the mRNA is read by ribosomes, and the information is used to assemble amino acids into a specific protein chain. This occurs outside the nucleus, in the cytoplasm of the cell.

So, while protein synthesis is a two-step process, only transcription happens in the nucleus.