Because the current government only have a majority due to a confidence and supply agreement with the DUP, who would break off that agreement if there was a bor

Because the current government only have a majority due to a confidence and supply agreement with the DUP, who would break off that agreement if there was a border between Northern Ireland and RoUK as part of the deal. Overall to Brexit supports in Britain it doesn’t really matter, it mainly matters politically for the current government. Without the DUP they would effectively be a minority government.