Several explanations: first, transitions are more likely to be silent than transversions and, if non-silent, more likely to connect similar amino acids; second, the transition bias in mitochondrial DNA is greater than in nuclear DNA; third, you will note that most of the changes are C-T, because there’s a base composition bias making G rare; fourth, transversions are often complicated by subsequent transitions that make them no longer informative for our sample — see, for example, the second to last site.