Was that the Amber series?
Of course, given that Narnia (and Middle Earth, for that matter) are imaginary worlds, the ultimate answer to any question "Why are things this or that way?" is "Because the author wanted it that way." But this only pushes the question back a level - why did the author want things that way?
Of course, we can only speculate on that, given that we can't ask Lewis (or Tolkien) directly. But having studied both men's lives and outlooks on life, I'd speculate there are a couple of factors that may have influenced their creating the worlds that they did.
One is very immediate and practical: they were both constructing worlds that exulted heroism and nobility. Weapons such as guns don't do that. In fact, many have observed that the advent of the firearm (and, interestingly, the crossbow before it) removed the element of personal risk and valor from combat. A weak, cowardly, unskilled person with a gun can kill a strong, noble, skilled warrior with a sword. This "leveling" effect has been lauded by some as a desirable development, but others (including myself) have observed that it had the effect of empowering bullies and cowards. In fact, you do find gunpowder in The Lord of the Rings - Saruman's troops use it at Helm's Deep (see the reference to "blasting fire"). It's seen as a coward's weapon.
A more abstract but fundamental reason may have been that Lewis and Tolkien were making a statement about "progress". The unquestioned assumption of our world - and the one implicit in prince_caspian's original question - is that increasing technological complexity is "progress". I think both these men, who saw the carnage of the trenches of WWI, seriously questioned whether technological progress, particularly in weaponry, was progress of any kind. To both of them, social "progress" would be growth in nobility, strength, courtesy, and humility. A culture wherein people watched out for each other and took care of the weak would be more "progressive" than one wherein people were selfish and the weak were victimized. This has nothing to do with levels of technological sophistication - you could have a simple society where people plowed with oxen and defended themselves with clubs, but valued nobility and honor, while another culture had telephones and automobiles but were rude and cruel to one another (or vice versa).
I think that if prince_caspian's question about technology "advancing" were to be posed to either Lewis or Tolkien, they would ask, "what does technology have to do with advancement?"
