I've asked this same question myself many times, mostly in different conditions since "my world" seems to be a little different than yours. But I will give you my piece on this matter.
Let's start with:
Is there are any way, assuming the world has progressed via a combination of industry coupled with magic, for guns to not really be a common weapon in war?
I'm assuming firearms do exist in your world but they aren't really common.
Combat "Inneficient" Kingdoms
In a scenario where the kingdoms are divided by specific powers, it would be only logical to assume that the concept of magic you're using is either bound to bloodline or specific studies not avaiable to everyone (in a worldwide perspective). This means that a given person can only learn telepathy, for example, if he's a member of the correct family or if he belongs to a kingdom that has deeply researched this branch of magic.
Unless you consider that most of these individuals (or at least a significant part of them) are overpowered (i.e: able to turn several people into marionettes) telepathy doesn't seem to be the type of magic that would be very efficient in actual warfare. Of course it all depends on your creativity. You could always say that the telepath kingdom can read every move of their opponents, making them formidable foes. But would they really be this good against an opponent capable of manipulating fire, ice or gravity?
In this line of thinking, it would only be natural that, for these people, an alternative solution could arise in the form of firearms.
Then we have this:
[...] more specifically, what circumstances - ie cultural reasons, lack of specific resources being available, etc - might have led to the traditional close-quarters army vs army combat remaining prevalent despite the technological advancement?
Lack of Resources
You kind of gave yourself a good answer here. You can just say that powder is really rare in your world - and thus, only a few kingdoms have the resources and the technology to produce it. This would limit firearms to big cannons instead of individual muskets to footmen.
The Mighty Generals
Another feasible explanation could surface if we come back to the overpowered people hypothesis.
The Lionheart Kingdom is populated with mages that can talk to animals. In war, every single soldier has an companion that has his back during combat. So a given soldier might run along a wolf or even a bear. This might not seem so powerful at first glance. But if you imagine a Lionheart general that fights alongside a huge direwolf, a dinosaur-like animal (assuming your world has a wild fauna) or even a magic-immune giant golem! That would certainly strike fear into the minds of their enemies.
This could apply to every kingdom. The pyromancer generals of the Firebrand Kingdom can cast a rain of flames upon their foes while the necromancer Lieutenants from the Tombstone nation can add the fallen in battle to their ranks amidst the combat.
The point is: If you have generals that can do such incredible things with magic, why use firearms?
Guns are useless against magic
This could be an extension of the Mighty Generals section. Would guns really be that effective against and enemy that can summon a thick ice wall around your cannons or under them, to mess up aim? What if your enemies can make their bodies as hard as steel?
Note that I'm not even using overpowered feats and they're already very effective against guns. The point is that magic is very wide, so there are multiple stuff that wizards can come up with to make firearms virtually useless or to drastically reduce their effectiveness. That being said, why should a kingdom ever invest in such a primitive form of warfare?
However, this hypothesis has a catch. If you choose to say that gunpowder is weak against magic for this reason, it makes this statement incoherent:
Crossbows and siege machines such as trebuchets are fine.
If mages can counter cannons, why would trebuchets work?
In my opinion, the exception here would be archery. Assuming there's no "wood Magneto" in your universe, a cloud of arrows would be highly efficient in battle.
That was what I could think of in the first matter. As for this:
[...] suggestions for how armies might be able to defend themselves against magic to allow for such large-scale ground warfare [...]
I can think of some stuff regarding that too.
For this, you would have to break the statement that magic is bound by blood/region - or, at least, that that is not true to some extent. This means that even though the shamans from the kingdom of Etherea specialize in contacting spirits, they could also have knowlegde in a more "basic" form of magic. This basic form could be crude magical power - "energy", if you need to label it - which, in turn, could be used to do basic stuff like projectiles or barriers.
So the shamans main skill would be to summon spirit avatars to aid them in battle, but if facing the terrible rain of flames from the Firebrand army, they could cast shields of energy to protect their troops.
You could always create some sort of charm, element, metal or whatever to make the wearer either invulnerable or highly resistant to magic. This is no innovative solution and if you want a quick demonstration, check the Snowdrop Flower from Stardust. Soldiers could use this suff in their vests while the very walls of cities could be imbued with such magic-resistent alloy or compound.
I like the idea of a flower (or some organic stuff) because you could always give a multitude of forms to this. Maybe the weapons of the soldiers are bathed in a toxin that suppresses magic or certain countries detain the method to build smoke bombs that have the same effect.
Hope this could be of any help. Maybe let me take a look after you're finished your book?