What do you mean by "perfect"? and when do you mean by "medieval China"?
Gunpowder weapons were regular elements of Chinese warfare from the 11th/12th centuries CE onward.

According to J. Lorge in The Asian Military Revolution, China wasn't that far behind in technology. As /u/lukeweiss mentioned, Chinese armies had been using all sorts of firearms quite effectively. Rifles, "shotguns" (starting with the firelance and later handheld shrapnel-cannons to defend walls). Even ships used cannons!

So why did they the Ming switch over to Portuegese and Dutch weapons? Because they were slightly better and it was easier to just buy them than to convert their production. The Chinese bought (and fished up) European cannons to study, but as trade between West and East increased, they decided to purchase weapons instead.

Lorge also addresses why the Chinese didn't use firearms against the 'mounted nomads of the North': soldiers armed with firearms were slow and unwieldy, unable to catch up to (let alone fight) highly mobile horsemen. Most of the time, the Chinese prefered not to go after the nomads and remained in fortresses. When fighting the nomads, arrows and spears a lot more effective.

In short: Chinese firearms weren't (that far) behind compared to European weapons up until the end of the 16th century. Their usage was mainly limited to defensive operations and battles with large armies: mounted nomads required different tactics and ships weren't strong enough to mount larger cannons. This meant that the Chinese didn't emphasize firearms like the Europeans did, but I've read European sources (which I cannot remember now, I read those over a year ago) which describe finely crafted Chinese cannons and guns which rivaled European equilavents. When the Europeans arrived, switching to their weapons was cheaper and easier for the Chinese.

Part of the issue with early handguns, and indeed most personal firearms until about the 1800s is that they had both an incredibly slow rate of fire, and were not exactly the most accurate of weapons.

Against continual raids by mounted nomads, who usually used bows, guns of the time were both far too slow and far too inaccurate to provide an effective means of defeating mounted archers, hence the Chinese continual use of crossbows and bows instead of guns as their rate of fire, higher accuracy, and comparable if not possibly longer range made them far more effective against nomad incursion.

Another issue with guns of the time was that reloading a gun while on horseback required either slowing your horse significantly, or was entirely impossible, both of which would make one incredibly vulnerable to the still very rapidly moving nomads with bows.

So in short, while the Chinese did indeed have firearms to use, they were simply not the right weapon for the kind of enemies they had to face along their northern border and so they stuck to alternatives which were more successful.

As I understand it, even though the chinese invented gunpowder much earlier and used it significantly in warfare, their priorities and military doctrine was different enough from Europeans that cannons weren't the most appropriate weapons for them.

Cannons work great in sieges, which were commonplace with the various castles and forts that dotted Europe. But early cannons are heavy, slow, inaccurate, and easily flanked, which made them pretty useless against fast-moving mounted archers like the mongols, who were the primary concern of the Chinese at the time.

Man-portable gunpowder weapons like muskets and rifles take a substantial amount of work in ballistics development and metallurgy to perfect (development the Chinese weren't really engaging in, instead building a big wall to block out horses).

There's also the issue that it took a while for gunpowder weapons to truly surpass the bow in effectiveness. For a long time their only real advantages were better penetration of armor, and it's easier to train a rifleman than an archer. But even that could be turned around into a negative. In Japan, for instance, while they made good use of firearms after their introduction, they were always more tightly controlled due to the highly-trained warrior class worried about peasant rebellions.

Disclaimer - not an expert by any means.

Another theory I have heard advanced is that the Chinese advanced bow technology significantly more so than was the case in Europe, meaning that it never paid off to decisively switch to gunpowder weapons in the same way that it did in Europe (we can also see this this in limited form in the West, where England was relatively late to replace its longbowmen compared to say France even though it wasn't exactly a technological laggard).

Of course the fact they were mostly fighting nomadic raiders as opposed to regular peer armies must have also played a huge role. That said, I'm not sure if that was the whole of it. By way of counter-example, unlike China, Russia was very enthusiastic about adopting gunpowder weapons, with the formation of professional streltsy units occuring in the mid-16th century. And by and large that didn't stop them from enjoying success against nomadic states. To the contrary, it marked a decisive rebalance of military power in its favor.