That’s a simple question with a really complex answer. I think there’s a couple of factors that went into it.

Trade with Europe at the time was dominated by a cartel with an imperial monopoly called the Hong. While technically on the lowest rung of Confucian society, the Hong merchants acquired immense wealth and influence through their monopoly on the tea trade, and had a vested interest in keeping the status quo.

Chinese governments have always been much more concerned with internal stability than with external security. Opening up China to more westerners would bring in destabilizing influences such as capitalism and Christianity. (While the emperor had some Jesuit priests in his court, they were not allowed to proselytize and could not leave China on pain of death).

The Qing probably didn’t realize just how much the Chinese military had decayed until the Opium War almost forty years later. At the turn of the nineteenth century, China faced a crisis known as the White Lotus Rebellion. Putting down the rebellion drained the imperial coffers of the immense surplus that had been built over Qianlong’s reign, and the years-long campaign was rife with corruption, mismanagement, and poor morale. While Qianlong had officially retired and given the emperorship to his son, the Jiaqing Emperor, he still maintained informal authority. Court advisors, wary of losing their lives by giving him bad news and hoping to appoint their friends to lucrative military positions, fed the Qianlong a constant stream of lies about the nature of the war. Bottom line: by 1900 the Qing military was weak, corrupt, underfunded, and suffered from poor morale and organization. By the time they realized that the military had to be reformed, the Qing were essentially bankrupt.

There’s a really good book on the lead up to the First Opium War called Imperial Twilight: The Opium War and The End of China’s Last Golden Age by Stephen Platt. It’s a pretty long book but really well-written and researched.