Mitochondria are the energy-making machines in the cell. Also, since evolutionarily they are thought to evolve from symbiotic bacteria, they have their own little genomes, which are mainly evolved through the maternal side, and also maybe because of their important function, mutations in genes affecting them are very damaging, and even lethal. However, mitochondrial DNA was not available to editing tools like CRISPR, so changes there were simply not possible, and any chance of recovering from such a mutation could only be either by approaching the symptoms, of from the womb via a three-parent baby with a mitochondrial donor. Now, however, mitochondrial gene editing is possible.
If you want to know how this is possible, check my last contribution to MappingIgnorance.org