Why do chloroplasts and mitochondria contain so many copies of their genome?

@article{Bendich1987WhyDC,
  title={Why do chloroplasts and mitochondria contain so many copies of their genome?},
  author={Arnold J. Bendich},
  journal={BioEssays : news and reviews in molecular, cellular and developmental biology},
  year={1987},
  volume={6 6},
  pages={279-82}
}