Because asexuality is the enduring trait of low sexual interest. You can still within that, have interest in a romantic partner, or romantic interest in a particular gender. Asexuality is more on sexual activity than romantic preference.

Next, pansexuality is the polar opposite in terms of activity, where it's 'gender-blind' or so they say. 'Omnisexual', so to speak, they have no preference, and love all basically.

I'd rather just keep it to the already well-defined hetero, homo and bi, as pan and asexual are still debated greatly.

Not to mention both are technically bisexual, and are considered subcategories at the moment.