super grossed out by the amount of people perpetuating that pan means "attracted to men, women, and trans people" as if trans men and trans women are not within that binary to begin with. uncool.
anyway, they're pretty much functionally the same word at this point, even if bisexual has the prefix bi-, most people who are aware of the existence of more than two genders and identify as bi include them, to my knowledge.
pansexual just literally means attracted to all genders, this means men, women, and nonbinary people of all types.
besides that, labels are personal, some people may not feel comfortable identifying as pan because they have possibly already come out as bi, have found solace in the term as they found their identity, or plain don't like the term pansexual. don't judge people for identifying as bi rather than pan because they're not being "open-minded" or assume that a bi person would not date someone outside of the gender binary.