Several reasons for this come to mind.

First, gunpowder isn't the wonder weapon that it's sometimes assumed to be. See this thread for a bit more info about that. Second, China's more troublesome borders were generally good areas for mobile, cavalry-based warfare, and the enemies the Chinese faced used these tactics. Firearms didn't offer many advantages over bows in that situation. (Nor did cannons, in the absence of a well-fortified enemy.) I know I've got that point from another thread on this board, but I can't seem to find it.

Thirdly, I think it's wrong to assume that the Chinese wanted to create an empire similar to Rome. Roman imperialism came out of particular forces and they don't necessarily apply to China.

First of all, Rome is a bad analogue for this, since historically China -did- create something like a Roman Empire which never fell before the gunpowder era.

China was unified in a process not at all dissimilar to the way that Rome unified the Hellenistic world, when the state of Qin conquered the other warring states, the Qin dynasty was short lived post-unification but the Han dynasty which replaced it held on to its territory and would go on to last for several centuries.

However, China was fragmented again when the Han dynasty fell, just as Europe did with the fall/decline of the Roman Empire. The difference is that China was reunified by the Sui dynasty several centuries later, and the subsequent dynasties were for the most part able to make this reunification stick. In a lot of ways China today -is- the East Asian version of a reunified Roman Empire.

Second of all, the Romans never used gunpowder, the better analogues would be the "gunpowder empires" of the Ottoman Turks, Mughals in India and the Safavids in Persia, all of which were able to use gunpowder to greatly expand their borders.

China during roughly the similar period actually did end up greatly expanding its borders under its Manchu rulers, but by and large China did not have the same room to expand as those empires. It is bordered to the west by steppes and deserts, north by tundra, east by the Sea and South by Sea and Jungle. The territories that China did hold was pretty close to what it could reasonably have conquered.