The mitochondrion isn't the bacterium it was in its prime,say two billion years ago. Since getting consumed by our common single-celled ancestor the "energy powerhouse" organelle has lost most of its 2000+ genes, likely to the nucleus. There are still a handful left—depending on the organism—but the question is why. One explanation, or say a mathematician and biologist who analyzed gene loss in mitochondria over evolutionary time,is that mitochondrial DNA is too vital to encode inside the nucleus and has thus evolved to resist the damaging environment inside of the mitochondrion. Their study appears February 18 in Cell Systems.
Source: phys.org