EDIT: but why would you need blackpowder when you have magicka that can do so much more, id rather roast my enemies to a crisp then try and blow them up. or in fact why need a bomb when a team of elite mages would have enough power to wipe out a whole town within minutes, without even going trough the gates?
Because for magic to be effective you need to train, to build a magic army you need to train many.
To lead an attack with bombs you just need people who can THROW them... far easier to get if you ask me. Plus a bomb can put you full of shrapnels.
Also for characters that are NON MAGES obviously.
but i doubt that its the type of cannon we are familiar with, its probably a device that amplifies a mages magicka when its cast on to it and that amplified magick then hurls the projectile into the air, but thats only a thought.
Ok, sorry but that sounds horribly like trying to "rescue" a point. They probably wrote it back then thinking it's not a big deal to mention cannons but now it just is in there.
Oh and btw. just for the ocasion people may say "It was just a JOKE, it doesn't count", it's in a IN GAME book meaning there must be a concept what a cannon is