Here's why:

* Multiple Alleles: There are three alleles (versions of a gene) that determine blood type: I^A, I^B, and i.

* Combinations: These alleles can combine in different ways to create the four blood types:

* Type A: I^AI^A or I^Ai

* Type B: I^BI^B or I^Bi

* Type AB: I^AI^B

* Type O: ii

The presence of multiple alleles for blood type leads to a greater diversity of blood types within the human population.