Well it was due to a multitude of reasons, primarily deriving from the failure of the T'ang dynasty to effectively suppress the An-Lushang's rebellion. His army was able to make the dynasty retreat from Xian which then appealed to the Uyghurs for help, who instead occupied the city and damaged it, refusing to leave until they were paid.

Beset by economic problems, another rebellion, the Huang Chao finally ended the time period where Xian was the capital.

The T'ang court did not move to Beijing, but rather to Luoyang. Beijing was the capital in succeeding dynasties such as the Khitan-Laio, the Jin, Song and Yuan.

The movement of captials for succeeding Chinese dynasties were dependant on who was around them. For the Khitan Laio and Jin, Beijing was close to their original territory and was an effective controller of the area. Captials often did move to the south of China as stronger Steppe based dynasties or Manchuria based dynasties had come into power.

Sources:

• T'ang China: The Rise of the East in World History by S.A.M. Adshead
• Soldiers of the Dragon: Chinese Armies 1500 BC - 1840 AD by C.J. Peers
• Imperial Chinese Armies Volume 2: 590-1260 AD by C.J. Peers
• The Cambridge History of Ancient China
• Diplomacy and Trade in the Chinese World 598-1276 by Hans Bielenstein

Adding to what /u/NomadicCircle said:

It was also moved away from the north to Lin'an in the 12th century. It was also previously in Beijing prior to the Tang period, in Chengdu and Chongqing in 1949, in Kaifeng a whole bunch, in Nanjing a whole bunch to include the 1930s, and many other places for other dynasties.

Basically, before your move it had also been in Beijing plenty of times, and after your move it was away from Beijing plenty of times. The capital moved frequently. And man other less well known and well known kingdoms in the area also often changed their capitals. Korea is not different. Japan is no different.

Beijing 北京 - northern capital

Nanjing 南京 - southern capital

Tōkyō 東京 - eastern capital (Japan)

Xijing 西京 - western capital, a name that's been applied to a few places

Kyōto 京都 - capital city (Japan)

Zhongdu 中都 - central capital

Gyeonggi 京畿 - capital territory (Korea)

Sometimes capital cities in the Chinese tradition have pretty straightforward descriptive names. Not always, but often enough for people outside the area to have noticed a trend.

Capitals change often. Names change often. Nanjing has also been called Jiankang, Yingtian and Jinling. Beijing has also been called Beiping, Yanjing, and, yup, even Nanjing during the Liao period.

It's less of a pattern than it looks today. It's mostly just a consequence of various shifts at various times in history, all relative to the situation at the time. Nanjing is far more north than other Southern capitals like Guangzhou.

edit: typoe