1) Regulations (vehicle fuel comes from sources/supply chains generally not willing to do the paperwork)

2) Lack of solid quality control relative to aviation fuels (see #1)

3) Power to weight ratio matters a LOT in aviation, and aviation engines run at typically far higher elevations for at least part of their flight time. So aircraft engines generally push things harder, have higher compression, and can be damaged more easily with 'junk' or contaminants.