Because regardless of what people today claim about demography changes, planting enough people in a country (England/Scotland --> Ulster from the 1600's onwards) will dramatically change the populace, culture, customs, and political affiliations of the region.
As a result, Northern Ireland has been populated by a majority of people of British (Primarily Scottish) descent for quite a long time.
The descendants of these people are:
Generally Protestant
Generally Unionist/Loyalist, which means that they will always vote to maintain the status quo that is the UK
Generally against the idea of a United Ireland in any situation, because they don't in any way identify as Irish
Because of Catholic birth rates, the population of NI is now closer to 50/50 Catholic/Protestant (Or Nationalist/Unionist, with Religion being less of a factor, the former being of Irish descent and the latter being of British descent).
Unfortunately, due to both time, the peace process, and in my experience as a Northern Irish person with everyone that I've asked, the existence of the NHS, we're no closer to reuniting Ireland as the idea has fallen to the wayside largely due to success of the GFA.
With Brexit though, the idea of a United Ireland has once again become a hotbed issue, with the majority still being in their camp of Nationalist = unite, and Unionist = don't unite.
Until something dramatic happens, this will likely remain the same way - as most people just want to get on with their lives and avoiding massive sweeping political and societal changes which may or may not be of any direct benefit to them.
On top of this, with a rapidly rising immigrant population over the past 10 years due to the peace in NI, we've now got a new voting population of people who really don't give a shit about the identity politics of NI and just want to get their wages and get on with their lives.
Personally, I'm in the camp of having always identified as Irish, and want a United Ireland above all else for the country - but losing the NHS is such a devastating side effect of the change.
In my opinion, the Tories privatising the NHS is the only thing that will dramatically swing the vote in regards to a United Ireland, which is ironic as they'd once again be damaging the interests of their own country for profit.
That or British troops putting their murderous, racist, Imperialist feet on our soil again post-Brexit. If a single British soldier saunters into the Bogside with a gun by their side again, you can be damn sure this issue is going to explode.