it was pretty much down to England wanting to retain a foothold on the island for strategic reasons. Reasons which at the time of the the GFA, the British Government conceded are no longer that important.
Ireland has not had any strategic purpose for Britain beyond the era of plantation. It contributed substantially less to the exchequer per capita than any other part of these islands, and was not geographically convenient to stage military posts to guard against foreign enemies (which came exclusively from the east and south, not the west).
It’s important to remember the actual act of partition kept - at that time - both states within the United Kingdom, though self-governing with dominion status.
The main reasons for partition were to end and act as settlement for the Irish War of Independence (though Home Rule had been mooted for some time before any violence), satisfy (unionist) violent paramilitary demands in the north, avoid the threat of army mutiny, and to break the deadlock of various Home Rule questions which had been swirling for decades.
The Government of Ireland Act 1920 (which created the original partition) was written with reunification in mind. Partition was not intended, at least in Britain in 1920, to be a permanent solution.
The GFA was not a concession by Britain to any strategic interest, but rather as a way of proving to republicans being asked to vote for it that the myth of “Britain is in Ireland for selfish interests” was just that - a myth.
The reality is the Government of Ireland Act 1920 always actively provided for, and envisaged, reunification.