Because unless the UK and the EU reach a trade agreement by the end of the year, the provisions of the Withdrawal Agreement kick in and Northern Ireland becomes bound to EU regulation.
This is a long and complicated story, but essentially Brexit created the following problem: the UK now desires to have full control over its borders, and this means a hard border between the UK and the EU. But then there is also the Good Friday Agreement (GFA), which achieved peace between the unionists (Protestants who want NI to be part of the UK) and the nationalists (Catholics who want NI to be part of Ireland). Simplistically, the GFA guarantees a sort of double affiliation of NI, both to the UK and to Ireland. This means, among other things, no physical borders.
This is a long and complicated story, but essentially Brexit created the following problem: the UK now desires to have full control over its borders, and this means a hard border between the UK and the EU. But then there is also the Good Friday Agreement (GFA), which achieved peace between the unionists (Protestants who want NI to be part of the UK) and the nationalists (Catholics who want NI to be part of Ireland). Simplistically, the GFA guarantees a sort of double affiliation of NI, both to the UK and to Ireland. This means, among other things, no physical borders.