Because medieval metallurgy stinks. While decent steel was available, it was extremely expensive. Decent, relatively inexpensive steel only appears in Europe around the late 1500 to early 1600s which is well into the Renaissance era.
One of the oft overlooked aspects of making a firearm, or almost any tool and weapon, is metallurgy. Making a chamber able to withstand the enormous pressures of an explosion in a very small space, plus the barrel to withstand all those hot gases, is remarkably difficult.
Black powder is relatively low pressure compared to smokeless powder, and the pressure wave was relatively slow. This required a long barrel to get the most velocity out of it. The low pressure meant your firing chamber could be relatively weak compared to what is needed with smokeless powder. So late medieval metallurgy was able to deal with black powder muskets and cannon.
Firearms, really hand cannons, first appear in China around the 13th century and show up in Europe and Arabian hands in the 14th. Around mid 15th century, the arquebus shows up, the first thing you'd recognize as a musket. This places only the most basic hand-held firearms solidly in the late medieval period. That's what your society would be capable of.
Regardless of whether it's black powder or a tiny magical fireball, it's the same pressure for the chamber to withstand. And it's the same lead ball going down the same inaccurate smooth barrel. So they'll have the same limitations. Nobody's going to have a Glock, magic or no magic, with medieval metallurgy. You get the equivalent of an early black powder musket, at best.
That said, there are reasons why you'd make a magical musket. Most of the innovations in muskets were to speed loading, and make the trigger more reliable. Magic can help.
- No powder to carry around and keep dry, just carry the shot.
- No powder residue to foul the inside of the chamber and barrel, a serious problem with black powder.
- Loading would be a bit faster, no powder to pour and pack in.
- No trigger mechanism to reset.
- No expensive and unreliable springs.
- No lit match. No flint to keep spaced just right. No touch hole.
Not having to deal with powder nor a trigger mechanism would be of great importance to a late medieval musketeer. Powder has to be carried, measured, and kept dry. Powder residue quickly builds up in the barrel and chamber requiring cleaning. The black powder residue could build up so fast in the barrel you'd literally have to use smaller balls if you didn't get a break to clean your gun in battle.
Trigger mechanisms will be the bane of firearms until percussion caps in the 19th century. The first would be a matchlock, literally carrying a slow burning rope to touch off the gunpowder. Then wheellock, using a spring to turn a wheel against a flint to throw sparks onto the gunpowder... hopefully. But wheellocks were very expensive, springs are expensive with medieval metallurgy. Finally the flintlock, a piece of flint striking steel, became the norm in the mid 17th century, but the matchlock stuck around.
It's not until the mid 1800s that we finally get the cheap, reliable percussion cap primers that we know of today: a small explosive that reliably detonates with a very hard whack.
If you told a medieval musketeer they could have a musket which required no powder, no match, no flint, and no trigger, they'd jump at it.