Kadaeux said
While Sini has part of it, the biggest part of it is simply that, Cannons and Musket are simply too overpowered. While magic (can be be much more overpowered) is limited to a few of talent, as the saying goes, any idiot can use a gun. Also, fun fact. The Roman Empire actually had invented a steam engine. They just didn't know what to do with it so it was a pure novelty.
No, any idiot cannot use a gun. Even modern firearms, though easier to handle and being of superior technological background, cannot be utilized correctly by any random smuck that chances upon one. Like any weapon, firearms require a certain level of experience, talent and finesse to use correctly and efficiently. While the firearm (generally) boasts more stopping power than the crossbow, shortbow or longbow, early firearms were prone to catastrophic failure, which in turn often resulted in the death of the operator or the loss of his hands.
You also need to ask yourself just what 'type' of gunpowder weapon you're working with. Gunpowder technology is not just limited to muskets. You also had early gunpowder weaponry dating all the way back to the 11th and 12th centuries, like hand-cannons, hand-held bombs (which were basically an early form of grenade), rockets (which later evolved into primitive rocket artillery, which was extremely effective during the era it was used), and mines (these existed, but were only effective when under the care of a man watching the mine, who would trigger it when an enemy walked over it).
Cannons ranged from gargantuan (and rather dangerous) brass-forged 'superguns' firing stone balls to the iconic smoothbore breech-loading cannons that we're all so familiar with. Mortars existed as well.
I'm not going to go into a massive history lesson here, but in order to seriously understand capability of the firearm and the tactics employed by those armies that chose to adopt it (most notable that of the early 'Pike and Shot' strategy and formations), as well as the different types of firing mechanisms later and more advanced smoothbore muskets used. The wheellock, matchlock and flintlock come to mind.
If you're worrying about firearms being overpowered, then direct your gaze to flintlocks utilizing rifled barrels, weapons firing Minié balls and flintlocks utilizing percussion cap technology. Combined, a weapon of this make can, in a sense, be considered 'overpowered' for a setting featuring mages and archers, as a man equipped with one of these and two weeks of training is more or less a force to be reckoned with.
In addition, if you're worried about the relative ease-of-use associated with the firearm, I suggest you remove crossbows from the game as well. They, like their gunpowder counterparts, required little training to use and could be given to conscripts withdrawn from the regional peasantry in times of conflict. They were also highly destructive (and deadly accurate), if not lacking in rate-of-fire. However, If used correctly, they were powerful tools.
There's also this: http://en.wikipedia.org/wiki/Arbalest
...Which is basically a crossbow on steroids and will gut a plate-armored knight and probably the man standing behind him at a distance of 300m. Rate of fire? Two bolts per minute (provided the user was seasoned and of sufficient strength and agility). Two shots per minute was about the same rate-of-fire an early firearm user could expect (though, this was probably truer for those that used early matchlock firearms like the Arquebus).
