For me it’s about verity, bows and arrows have been done a tone, and of course the 'vibe.' Guns, weather specialized magic wands or not, are a major part of the world and its history. Various types of gunpowder biased weapons have been around for a lot longer than many would expect. And it’s kind of weird to see plate armor from the same age as the first siege canons but lack the weapon all together.
Then it’s the fact that magic makes heavy armor viable against firearms. And of course, cannons, musket lines, and all kinds of similar things are just very cool to play with, and when you have knights in heavy armor going up against line of muskets, and mages throwing balls of fire everywhere. It’s just fun.
And of course, cannons have their place in medieval fantasy having been used in the hundred years war by both sides. My memory might be wrong, but I'm pretty sure Joan of Arc would have fought alongside cannons as well.
Basically, for me: It just fits well enough that overlooking it seems deliberate and stands out. Especially for when cultures heavily use stone walls and massive castles.
Plus, it fits so well as an invention of classical orks that I'm a little disappointed that games with strong lore and gunpowder don't have it invented by them. Of course, this isn’t a complete focused on AOW4, more AOW3. Personally, I’d have built that campaign a lot differently and put the most dreadnought focused race as the orks. But personal preferences are always different.