Because they’re our cousins up there, they’re Irish citizens, it’s traditionally our territory, and 100 years of unionist (mis)rule up there doesn’t mean the status quo is somehow acceptable.
The concept of NI only originated a hundred years ago. Its borders were drawn in such a hasty way and an ad hoc fashion that both parties to the Anglo-Irish Treaty agreed they would be changed within five years (but that never happened). It’s literally there in the Treaty. Thus NI is an absurd, artificial, gerrymandered statelet. If there were an all-island vote tomorrow, NI would cease to exist. It’s madness.