Here's why:

* S phase stands for "synthesis" phase. This is the stage where the cell replicates its DNA, ensuring each daughter cell receives a complete set of genetic material.

* Organelle replication is tightly linked to DNA replication. Many organelles contain their own DNA (mitochondria and chloroplasts), and this DNA must be replicated as well.

* The replication of other organelles like the endoplasmic reticulum, Golgi apparatus, and ribosomes is also coordinated with DNA replication. They are often synthesized from pre-existing components, and their growth and division are controlled by signals triggered during the S phase.

While most organelle replication occurs during S phase, some organelles, like mitochondria, can replicate throughout the cell cycle as needed.