Because it will seem grander by doing it at D23. By doing it in a event like they did with nightastic, it could go unnoticed by a lot of people. But this is on a much larger platform and could catch the attention of lots of people that otherwise would just let it slip by if it was done like nightastic was.
I agree. But, for most of the Disney Population, the D23 Announcements and the Nightastic Annoucements were the same thing. Us, gathered around our computers, on forums, on Twitter, waiting for the news to break.
The loss here, is the fact that a few people could see this first hand.