"Jack"in the dictionary also means "ship's flag flown to show nationality".
Because Northern Ireland,England,Wales and Scotland joined together to become the United Kindom and this was shown to the world in a new flag called the Union Jack.Besides,the Union Jack was designed based on the flags of the four countries.All the crossing on the four flags were joined into the Union Jack.You can find either flag of the four countries in the Union Jack,so to speak.
Now you can see why the Union Jack is called "the Union Jack".