Because, as I understand it, it will contravene a clause in the Good Friday Agreement and the various dissident IRA groups, which are already active and bombing and killing will use this as a justification for saying that the Good Friday Agreement has failed and an excuse to increase their murderous activity.
I think this will be so because, although the Good Friday Agreement is actually very vague about the border. To quote a BBC source www.bbc.co.uk/news/uk-northern-ireland-46988529 During the Troubles there were heavily fortified army barracks, police stations and watchtowers along the border. They were frequently attacked by Republican paramilitaries. Part of the peace deal involved the UK government agreeing to a process of removing those installations in what became known as "demilitarisation".
It could be argued that if a hard border was restored it would be manned by the Border Force and I am sure it would not take the dissident IRA groups long to claim that the Border Force are effectively a para military force and the border was again 'mililtarised' and use this as the excuse for increasing their activities
Would the main IRA, led in the past by Martin McGuiness and Gerry Adams, immediately become active again? Probably not immediately although some members may collaborate with the dissident groups. But the IRA still exists, still commands a lot of support and is still committed to the re-unification of Ireland at the barrel of a gun if other means take too long.
All this is not helped however by the continued lack of a viable government in Ireland, for which both Sinn Fein and the DUP are responsible, but each for different reasons.