Is it really any surprise that 3rd party characters are typically saved for the events that'll have millions of eyes on it?
It's really not that strange. Historically, Nintendo characters have either been in normal Directs, OR come alongside 'big' reveals. This more or less held true for E3 2014 as well, where even though Palutena was revealed, we had Miis, which were being touted as this big new feature of "Now YOU can be in Smash", and later that day, Pac Man was also revealed.
Now of course, this isn't a pattern by any means, Cloud and Terry were revealed in a standard Direct, but Cloud was also used as a hook to announce "HEY, LOOK, WE'RE DOING A BIG, FINAL PRESENTATION! CLOUD'LL BE THERE, YOU DON'T WANNA MISS IT!"
And as much as I love Terry.... he's definitely the smallest 3rd party we've gotten this cycle. That's not bad by any stretch of the imagination no, but he's definitely the smallest of them all, and probably the absolute cheapest to license if the sheer amount of content he came with is anything to go by.
Of course, this also works the other way as well.
Ridley, a 1st party character was revealed at E3 2018 with no 3rd party character, but at the same time, he was also like, the de-facto most wanted character, period, and alongside him was "EVERYONE IS HERE!"
There's no 'rule' that 3rd party characters are more likely for big events, but I also think Nintendo realizes that they'll get more eyes on them for "John Mc3rd party was shown off at the biggest gaming convention in the world" moreso than "We're adding another Nintendo character and going to use this once a year event to show it off."