Sure. So just to get some definitions out of the way:
Bisexuality: The attraction to your own gender and different genders.
Pansexuality: the attraction to all genders.
Obviously there is a lot of overlap there. MANY bisexual people, myself included, are attracted to non-binary people, and to suggest that we aren’t as a rule is very inaccurate.
Why people identify as bi or pan is often a very person thing and involves your relationship with those terms and communities.
In general, bisexuality has a longer history as a political force, a longer history of activism, which I personally find appealing. Also, as a bisexual, I feel that gender DOES in some ways impact my attraction to people. I find different traits attractive in different genders, so while I am attracted to multiple, gender does play a role.
Pansexuality is a newer term and a younger community, which some people find appealing. Also, many pansexuals describe their experience with attraction as having nothing at all to do with gender. Many pansexuals describe this as being “gender blind”, where their attraction to someone has nothing at all to do with their gender.