China has made scientific and technological achievements leading the world. Why has it been invaded by foreign powers repeatedly since modern times?

When you say that China has the world's leading technology, you mean before the Qing Dynasty. Specifically, the Song Dynasty was the era when China's technology reached its peak. It was ok in the early Ming Dynasty, but in the middle and late Ming Dynasty, the development of science and technology was basically stagnant because of internal troubles and foreign invasion, such as firearms. At the end of the Ming dynasty, the firearms equipped by the army were almost the same as those at the beginning of the Ming dynasty, and the artillery was imported. The rulers of the Qing dynasty were short-sighted, did not pay attention to the discovery and invention of new technologies, and closed their doors to the outside world. In addition, the rulers of past dynasties attached importance to agriculture and restrained commerce, so the small-scale peasant economy in modern China was self-sufficient and the capital could not circulate normally in time. The no demand nature of new technology eventually led to the weakness of modern China's national strength and was finally opened to the outside world by western powers.