It was rejected because you mis-defined the 20th century as 1900-1999. The 20th century was actually 1901-2000.
Your proposed tag wiki says:
For questions relating to 20th century English, i.e. 1900-1999.
If you had typed in 1901-2000 instead of 1900-1999, it would have been approved.
Update: status-completed, as I just implemented a new description ...