Because fantasy world doesn't have to follow the same one of a kind cultural processes like Europe (with 'neighbor' civilizations that were involved in big melting pot of inventions and ideas).

Look how differently Europe and South America looked like ~ 1550

Gun powder is particularly 'easy' - culture have plate armor or similar stuff, but lack of interest in alchemy, lack of experiments with potassium nitrate (and lack of such) and other individual occurrences - and BOOM (hehe) no guns.

using Viking-age swords and shields tactics when they have such fine armour (generally, it's because the armour is made out of tinfoil, but that's beside the point).
Shield was still used very plenty by infantry in 15th century, even though it somehow felt out of use in 16th.... not quite "Viking age" but that's really detail.

Tight shield formation still offers interesting options, particularly for poorly armored infantry, especially against opponent with many missile weapon.