https://www.scientificamerican.com/article/why-do-people-have-differ/ https://www.scientificamerican.com/article/why-do-people-hav... for a decent lay-treatment

https://www.scientificamerican.com/article/why-do-people-have-differ/ https://www.scientificamerican.com/article/why-do-people-hav... for a decent lay-treatment of the question. One important thing to remember is that on an evolutionary time scale, the problems that arise from transfusion just started happening. There's been no time to select for them. Instead, different blood types have different selective advantages that help maintain them, and incompatibility isn't sufficiently detrimental to offset those advantages.