Without going all AK47, early firearms generally run into the problem that their historical rate of fire is nowhere near the number of attacks that D&D characters can have per round, especially considering fighters. So firearms in D&D either go the way of re-fluffed crossbows and hit a rate of fire unmatched until the 19th century, or go the way of "magical attacks" with single high-damage effects and become essentially non-weapons for the purpose of abilities, feats, etc. Expectations can be hard to reconcile with the rules and that turns some people off.