Why do you think people has different type of blood?

because of nature and its natural selection. different people have different blood types just as different people have different hair style (natural), or different nose, diameter of mouth, ears, etc. but at the same time, we inherit it from our parents. using the punnet square, a A+ mother with an O+ father can have children having A+ or O+. In a more complicated aspect, a heterozygous AO+ mother with a homozygous BB+ father can have childrens having AB+ or BO+ again using the punnet square :) blood types are determined by the type of antigen they have in their blood. A type A individual has A antigen and B antibody, a type B individual have B antigen, and A antibody, and so on. this is the reason why there will be transfusion reaction when a B+ individual is transfused with an A+ individual because the antigens will react to the other blood's antibody. im a medical technologist by profession, hope i helped :)

Thats like asking why are all humans different.