The immediate factor was that the Qing dynasty had more or less not had to fight a major war for almost a century by the 1840s, and the British had military technology at least few decades ahead of the Chinese. The Qing military had grown lax and limited effort was made to modernize it because there were no major wars to fight. The result was that Qing banner armies performed abysmally not just against British forces, but also against domestic Taiping peasant rebels later on in the century. Whereas in Europe, the competing state system meant every state much maintain the most modern army and navy no matter the cost because the alternative is the destruction of the state.

The deeper factor is that the Qing was really facing a crisis of governance in the 19th century. Between the beginning of the dynasty and 1800 the population of China and quadrupled without a corresponding increase in governmental capacity. What this meant was that the Qing government was unable to squash rebellions in its infancy nor to more effectively implement modernization programs. Both of which left China very vulnerable to western powers, there's also a kind of "snowball" effect where every time China lost a war, it had to pay massive amounts of indemnities which strained the already fragile resources of the state, which in turn means that it had less resources to conduct modernization programs, which in turn meant it became even weaker vice-verse the west etc.

Finally, I think it should be noted that China did not necessarily do badly in the era of colonialism. A Chinese state capable of fielding a modern navy and army existed by the 1890s. The Qing government was also effectively able to adopt to the Westphalian model of international diplomacy and retain sovereignty over most of its territories despite incredible amount western and later Japanese pressure.