Because it was the flag of the Beiyang government in Beijing, and the people who unified China in 1928 were the Nationalist Party which was based in the south and wanted to make the capital Nanjing. Originally, in 1912, the Nationalist Party was included in the Beiyang government, but that relationship fell apart in 1916 when President Yuan Shikai (who was opposed to the Nationalists) declared himself emperor.