Why does the 1921 Census not include Scotland and Northern Ireland?

In Scotland, National Records of Scotland (NRS) takes responsibility for historical census data. Scottish 1921 Census records were collected on the same night as those in England and Wales but processed and stored separately. The digitised records from the 1921 Census returns for Scotland are available on a pay-per-view basis at www.scotlandspeople.gov.uk and in the ScotlandsPeople Centre. 

As with Scotland, Ireland and Northern Ireland conducted their own censuses separate from England and Wales. However, The 1921 Census was not taken in Ireland due to the Irish War of Independence. Censuses in Ireland and Northern Ireland were conducted in 1926.