Cap'n Kobold
Hero
If I remember rightly, making a gun is much harder than making gunpowder. Gunpowder had been around for quite a while before advances in metallurgy (driven by demand for church bells IIRC) allowed the creation of an actual effective firearm. Making a metal tube strong, straight and smooth enough to send an effective projectile down is the hard part, particularly for a tech level that has stopped developing metallurgy at swords and armour.for the exact same reason guns replaced bows in real life: logistics.
even in settings with reliable magic, the general assumption is that learning magic is really hard and takes a long time (kind of like training to effectively use a longbow). learning to use a gun is neither.
now, learning to make a gun? that's a bit of a tossup, but i think it's relevant to note that gunpowder was (allegedly) originally created as an attempt at an elixir of immortality (i.e. a failed alchemical experiment). that sounds like classic wizard/artificer shenanigans to me. my guess, though, would be that making normal guns would probably be less difficult and expensive then making rune guns (and, by the stats you've given for them, absolutely more efficient - 7 shots a day max without specialist attendance is flat out pathetic, even for an arquebus or hand gonne).
or, you know, you can go the zeitgeist route and say "because COUNTRY SIZED ANTIMAGIC ZONE!" that works too, i guess.
This is why rune guns may well be cheaper to make, because the knowledge and capability for them might already exist, whereas that for firearms might not.