Because that's not how they do NSO announcements. Individual NSO drops only get featured in Directs if there's a new platform or the game is a really special case like EarthBound. NES/SNES/GB updates mostly just appear at random with no warning like this. One time they decided to give a new roadmap for N64, but that's not what's happening here.If a Direct is indeed forthcoming, why not announce these NSO updates on the day? We've already had updates announced in other Directs, they could have waited if that were the case.
Nintendo likes to do a lot of their announcements in Directs, but not all of their announcements are Direct material. Everybody's 1-2-Switch falls in a similar category for different reasons.
And just to preempt this, in case they decide to do a June one this year, Pokémon has their own version of Directs called Pokémon Presents that they generally use instead of Nintendo Directs these days. At one time Pokémon games were Direct announcements, but they haven't been for a while.