Part of the reason we have so few blood types is because humans nearly went extinct during the ice age. I think estimates put around 2000~ humans alive at one point. Because of this massive bottle neck, humans of today arent as diverse as you find in other species. It also explains why we don't really have much sexual dimorphism in comparison to other species as well. Except, species that also went through similar near extinction events, for example; Hyenas. They also went through a similar near extinction event and as a result have a ton of associated diseases having to do with it. Just like us.

This is useful because it means animals we drive to near extinction will be altered for thousands of years onward even if we were to vanish right now.