Law differences (hello, abortion), possible (though obviously not certain) discrimination - ie, only those who speak Irish are allowed to teach in Ireland (certainly at primary level, not sure about higher) - and also political. 15% of Catholics reportedly wanted to leave the EU compared to 60% Protestants.
There is also, and I can't stress this enough, the cultural divide mentioned previously. "Why doesn't England/Scotland/Wales enter a union with ROI?" can fairly succinctly be answered by "they don't want to, even were there benefits - they are not Irish." Northern Ireland, as well as this cultural divide, has what Grim called "friction left over from the Troubles," which is an understatement. Protestant families see a past stained in blood, where the union was challenged and persevered. It is unlikely that this will casually be given up, no matter the reward. From personal experience, they
are unionists, and loyalists, in that in many cases there is loyalty to the union, something perhaps not seen so often in contemporary politics. When you add that to the blood toll (something very personal, linked to local places where family friends died) it adds to a large sentiment of "I'm British. I am different from those in the Republic. I would not be represented properly. I do not want their institutions, or their religion. My friends/family died in this vein - I will vote in it."
Funnily enough, a "reunification" would incidentally see the Republic joining the UK again, given that theirs is the separatist state