This is a very interesting article, but the core question is completely wrong-headed. There is no “why”. We just do. Evolution is as much about errors in the replication of DNA as anything else; some errors convey serendipitous advantages, others disadvantages. Some make no appreciable difference. Blood types fall into the latter category, having simply arisen as variations which had no impact on successful breeding or survival. Until the advent of blood transfusion techniques, blood type would seem to have had little if any consequence for an organism whatsoever.