Firearms occupy this strange space in gaming. Some people are convinced that such weapons would instantly destroy the faux medieval setting with an industrial revolution, make armor useless, and obliterate monsters left and right. The reality is, full plate armor and cannons didn't exist until the 1300's in our world, we had hand cannons in the 1200's, and both the bastard sword, arquebus, matchlock, and the rapier were all invented in the 1400's.
The limitations of early firearms make them very difficult to use in the short skirmishes D&D is known for. Conversely, if you add firearms to a game and they aren't immediately superior to other weapons in all respects, some people will reject that outright! It's somewhat akin to the "katana madness" that once infected the gaming sphere where people who have bought into the mythology surrounding a weapon insist that it should be reflected in the game's mechanics.
All of this is quite silly when you consider D&D, in particular, which in the past deliberately made certain weapons strictly inferior to others based on designer bias and wanting to elevate certain forms of combat in the game- look at all the various times weapons that were once feared on battlefields are barely worth using over the years, like slings, crossbows, warhammers (really Gary, d4+1?) and so on.
It's especially amusing when one considers siege weaponry, since the Magic-User was originally designed to be an analog for such weapons on the battlefield!