Why is O positive the most common blood type?

Answer 1

blood types are linked with our ancestry. Dr D'Adamo states that our digestive tract retains the memory of ancient times, and so type Os need to eat a typical hunter-gatherer type diet. In other words, type Os should follow a high-protein, low-carb diet with lots of meat and fish but no dairy products, wheat or grains.

so to answer your question most peoples ancestral tree ate meat, leading them to have O blood type