Because that's not actually what it does.
A nocturnal creature is not a darkness creature.
Holy light doesn't affect nocturnal creatures - it affects death and darkness creatures.
Anyway, it's being strongly implied the two elements I'm using are not ones i should be, so what would happen if I switched around some of the elements and came up with a different effect?
Originally, the card was a darkness card pretending it was a light card, giving death and life creatures +1 | +1.