Why did ulster remain a part of great Britain in 1921?

Northern Ireland, not Ulster, remained a part of the United Kingdom in 1921, when the rest of Ireland became the Irish Free State. This decision was made following the Government of Ireland Act of 1920, which partitioned Ireland into two self-governing regions: Northern Ireland and Southern Ireland.

Northern Ireland remained part of the UK primarily due to the wishes of the majority of the population there, who were predominantly of Protestant and unionist background. They wished to maintain the strong political, economic, and cultural ties that existed between Northern Ireland and the rest of the United Kingdom. The Government of Ireland Act included provisions for a potential union of Northern Ireland with the Irish Free State, but this was never pursued due to the opposition of the unionist majority.

Furthermore, the border between Northern Ireland and the Irish Free State was drawn in such a way that Northern Ireland included several counties with significant unionist populations, which helped to solidify its position within the UK. Despite some objections and attempts at boundary adjustments, the border has remained largely unchanged since 1921.

In summary, the decision for Northern Ireland to remain part of the UK in 1921 was driven by a combination of factors including the wishes of the majority population, the existence of a unionist-majority government, and the border arrangements determined by the Government of Ireland Act.