China just didn't have the technology, they didn't even have Euclidean geometry or the Archimedean screw. It wasn't one thing or even a few things, they lacked a cluster of innovations developed during the renaissance and dating back to the classical era, both what modern minds understand as technology and mathematics but also innumerable smaller innovations used by artisans.

They were too far behind to manufacture flintlocks themselves and did not like the idea of becoming dependent on European imports, so the Qing in its heyday just cut off contact with the outside world. It was difficult for 17th century Europeans to develop the flintlock which was both reliable and able to be produced cheaply enough for the ranks of levied peasants, as opposed to some rich cavalier's fancy wheellock pistol only intended to be fired once before being handed to a squire for the laborious reloading.