Because that was what was decided by the game's creator and there's no real *gain* for the change. There are plenty of things that I or Anaris would love to change ourselves, but do not, because it is Tom's game and there are certain things he has decided. This immediate change of monsters/undead bandit/highwaymen flavour text is simple. But scrolls are low fantasy, so you'll want them gone too? And...where do we stop? See, its not so simple if we keep applying the idea.
BattleMaster derives from an older email-based game, SpellMaster, that involved wizards, magic, and spell councils. It's just a part of what BM's history and what BM is. But, for what its worth, BM was founded as *low* fantasy with SpellMaster providing the *high* fantasy side.
EDIT: Also, this is not what BM as a game is aiming towards - "medieval army commander simulator"