Why can't I challenge NPCs to Triple Triad?

So, I've noticed on occasion certain NPCs will say they're too busy and decline a Triple Triad match. Any idea what determines this? I first saw it with Tataru and figured it was due to quest involvement, but now I'm trying with Trachtoum, whose only quests are long done with.

Some NPCs can only be challenged at certain times of day

It's a way SE time gates you from content without actually restricting you from it, basically.