There is some sort of official in-game answer, in the Forgotten Realms Adventures (FRA) published right after the update of AD&D onto 2nd edition. On page 11, under the section "New Weapons - Firearms in the Realms", the introduction of smoke-powder (a semi-magical version of gunpowder) weapons by the Lantanese, whose state religion is the worship of Gond, is discussed. These new weapons (arquebus, musket, blunderbuss, etc.) are expensive, can fire only once every 3 or 4 rounds, and are not much better than arrows shot from a longbow in terms of damage dealt. Furthermore, there is a 10-15% chance of backfiring. So you can consider them as rather primitive devices.
In the real world with no magic, one can imagine that there will be incentive to improve those weapons. But on Faerun, who needs to do that? Actually quoting directly from FRA:
Firearm technology has never been extensively (or even adequately) researched and developed, however, save for a few crackpots and eccentric wizards. The reason is simple - who needs firearms in a world with fireballs? (The answer, of course, is people who can't cast fireballs.) No major nation or organization has invested time and money into producing of smoke powder weaponry on a large scale.
So, at least as far as weapon technology goes, the official answer is the second reason you mentioned in your question.