Because her husband Gaozong is not a strong emperor. Gaozong was not interested in politics. Sometimes, he asked Wu Zetian to cope with the political affairs. Moreover, Wu Zetian is a woman who liked rights very much. She was smart and capable. She killed the ministers that prevented her from getting power and nurtured her own supporters. After Gaozong died, she became more and more powerful. In order to seize the crown of emperor, she even killed her two sons. Her another two sons were talentless. Especially, her third son Li Xian was very incapable. In the war of seizing imperial power, she won. She changed the national title 'Tang' to 'Zhou'. However, when she became emperor, she had been old.

It's very difficult for a woman to become an emperor in a male dominant society. The process of being an emperor is very brilliant. However, once you became emperor, you would realize it's not better than the process. The war of seizing rights is full of blood and death. Maybe, the women after Wu Zetian had known this truth. So, they didn't want to be emperors.

As a women, actually Wu Zetian wouldn't like to lose sons. But she had walked onto the way that leads to the crown of emperor. Once she gave up, many of her enemies would kill her.

All rulers were originally warriors, and few women were warriors before very recent times. Men would not follow them. There were no female emperors of the Roman Empire (for example) until 797 A.D., when Irene seized the throne by putting out her son's eyes. There was never a woman emperor of the Holy Roman Empire in the West either, and that lasted a thousand years. There was hardly ever a woman pharaoh. There was never a woman reigning as king of France - only Queen Consorts and Regent Queen Mothers. It is always unusual to have a reigning woman - as long as the monarch has actual power. Once the monarch becomes a merely symbolic figure, women sovereigns are frequent and popular.

there r many other great women who had once ruled the country ,but their positions were not permitted by the history

to ms.bester
ya there were many other great women who ruled but they did not proclaim themselves as empress regnant. they only ruled as either empress consorts or empress dowagers. they only ruled 'behind the curtain'.