MitoQ is marketed as a supplement in order to make it available to everyone without a prescription.
If it was registered as a pharmaceutical, its approval would be locked into certain health conditions, and a series of large clinical trials would be required to get it registered for every different indication that it showed promise for. This would possibly then need to be repeated in different countries. Since MitoQ does appear to have a wide range of benefits, marketing it as a supplement is the best way to make it available to everyone. Incidentally, MitoQ was initially developed as a pharmaceutical but this route was discontinued early on for the above reasons. A growing body of scientific research continues however, and you can find these studies on Pubmed or clinicaltrials.gov if they are of interest.