Confucian ideology holds several things to be true:
1. Merchants are the lowest of the low: they produce nothing and profit by moving someone else's products. The number one motivation for mainland empires to develop strong navies is to safeguard mercantile fleets and secure the local trading zone. This requires organized mercantile activity conducted with the participation of the state, like the VOC in Holland and the British East India Company in Great Britain; having the trade be conducted by foreign powers is not conducive to this because if they're making the effort to get here, they might as well protect their own damn ships, especially if they're not essential to China's economy and hence not something worth defending!
There's also another problem: mercantile activity requires a degree of independence on the part of the company, which the Chinese bureaucracy will never allow. If it's not under the bureaucracy's thumb, the bureaucracy won't allow it.
2. China is the Middle Kingdom, the center of the universe. The default position for everything outside of China is that it's inhabited by barbarians who may or may not have interesting things, and the default position for those barbarians is on their knees, sucking China's dick and providing China tribute. China is automatically the best land in the world, so there's really not much point in straying very far outside its borders... unless you make some very significant changes to the state ideology. By the time that the rest of the world can really prove this attitude wrong, it's already way too late to make any colonies.
3. China's bureaucracy is a graft-ocracy from top to bottom, because the low-level bureaucrats don't get paid very well while also having boundless opportunities for corruption and bribe-taking. This was remarked upon with great scorn by many Western explorers and travelers to Qing China as universal.
4. Confucian bureaucracy is very reactionary by design, and they can often overpower the emperor's will despite him being an absolute monarch on paper.
The first problem here is that the Confucian bureaucracy also doubles as the official historian class of China, which means that they have the final say over whether emperors (and their ideas/policies) are judged as good or evil in the history books. Go against the bureaucracy and you'll get your wishes done for a time, but your successor will probably reverse your policies, for which he'll get to feature in the histories as the righteous restorer of order and tradition.
For an example of this effect in action, look up "The Wanli Emperor's strike", where the Wanli Emperor, faced with a recalcitrant bureaucracy, did the only thing he could do: stop fulfilling his functions.
The second problem here is that the Confucian bureaucracy always wants to return to an older state of being; if new reforms are implemented, they'll only be maintained as long as there is official force backing them up and the situation requires them to be in effect.
For an example of this effect in action, look up what happened to the Yongle Emperor's ambitious naval programme after his death: cancelled.
Often enough, reforms would be sabotaged because the bureaucrats involved were too busy looting the budget:
1. Example: what happened with the Hundred Days' Reforms. That was stopped by Cixi in coordination with a cabal of officers and bureaucrats disaffected by the changes.
2. Whole segments of the fleet modernization budget were reappropriated by Cixi so she could build a palace for herself.
3. Example: the courtier
Heshen, a favorite of the Qianlong Emperor, ended up embezzling almost the entire treasury (!!!) and could only be removed once the Qianlong Emperor died because the Emperor wouldn't hear a word said against him. Even after Qianlong abdicated, his son still couldn't do anything about him because he still had his father's ear. Fortunately, once his father died, the Jiaqing Emperor was able to get shit done.
Moreover, that older state of being can be quite problematic for imperial expansion.
For example, China proper has historically had very low tax rates. This was a state of affairs which the emperors were expressly forbidden from changing, which often forced them to look outward in order to supplement their taxes, forcing them to acquire and exploit their vassals. Notice that this reality goes hand-in-hand with the idea that the correct position for China's neighbors is as tributaries, not as equals. Same happens in every dynasty.
For example, the Kangxi Emperor was thus forced to subjugate the Dzungars and levy heavy tribute from them. This later caused them to rebel, for which the Qianlong Emperor "pacified" them, which in Chinese bureau-slang means "genocided".
This is but the latest in a long round of such genocides, going all the way back to the Tang Protectorate-General to Pacify the West and their genocide of the Gokturks; it was what originally started the Turkic migrations to the west, first after they destroyed the Eastern Khaganate in 630 and later when they destroyed the Western Khaganate and genocided its people in 657.