I’m assuming it’s because Zeraora is a mythical and Zygarde is a legendary and both probably had mega forms in mind given that Zygarde was supposed to be in Gen 6, though Zeraora might be the gen 7 rep for Megas.
I meant they might do a couple more regular Pokémon (especially now that Gen 9 is out) but they main focus will probably be on those two legendaries getting Megas. Hopefully they add the scrapped Flygon mega and more.
They probably only planned for a mega Zygarde in terms of legendaries, unless they pull an ORAS on us. Hopefully they planned more normal Pokémon legendaries.