Alright, there's two main reasons:
- Night Daze is 95% accurate instead of 100% accurate. It might not seem like much, but that certainly is a reason.
But more importantly,
- Night Daze is Zoroark's (and Lunala's) signature move. Let's say Zoroark, transformed into something like a Greninja, uses Night Daze. The opponent is immediately going to know that it's a Zoroark, not a Greninja. If you use Dark Pulse instead, the opponent may not suspect anything and play around Greninja instead of Zoroark.
Of course, reason 2 may be incredibly niche, so if you're just using Illusion to get a free turn to setup or something that doesn't involve attacking, Night Daze would be better (accuracy willing). Smogon does list Night Daze as an alternative to Zoroark past generation sets. However, as power creep grows, Zoroark NEEDS Illusion to provide as much value as possible, meaning it can't risk Night Daze giving up it's disguise.
And of course, Lunala never uses Night Daze.
Therefore, it's mostly an issue of preserving Illusion, as most Zoroark users would take the minor base power decrease to make the most out of their ability.
