Why do mitochondria and chloroplasts have small genomes?

The mitochondria and chloroplasts in eukaryotic cells are descendant from bacteria. Ancient eukaryotic cells ingested these bacteria and they became part of the larger cell structure. Mitochondria and chloroplasts work symbiotically with the cell to provide it with energy. Because these mitochondria and chloroplasts used to be separate bacteria living on their own, they had their own small circular DNA like all bacteria do. When they became part of the eukaryotic cell they retained this DNA. Mitochondria and chloroplasts also reproduce similar to bacteria and replicate their DNA the same way. In this way, all mitochondria and chloroplasts in the organism contain the same circular small genome, just like all nuclei in the organism contain the same spiral DNA.