Why?
You need to understand the political history and the geography. Geographically Northern Ireland is part of the island of Ireland, it isn't part of Great Britain. Great Britain is the largest island within the British Isles, and comprises Scotland, Wales and England. Note the island of Ireland forms part of the British Isles, together with the Isle of Man, Inner & Outer Hebrides, Shetland and Orkney.
Politically/historically Northern Ireland is part of the United Kingdom, which comprises the Kingdom of England and the Principality of Wales (1284), Kingdom of Ireland (1542) and Kingdom of Scotland (1707). However, southern Ireland left the United Kingdom in the act of partition (1921) becoming the Republic of Ireland; the northern provinces voted to remain part of the United Kingdom and are referred to as Northern Ireland.
.