The next break year inst scheduled until 2017 or 2019. And we know there was a game in 2017.

Also it was officially stated "2018 or later" so 2018 was a possibility

And when it was announced in June, they officially said "it may not come for more then a year"...

December 2018 is "more than a year", and it would give time for them to reveal Zeraora in February, then announce Generation 8 March with a 10 month gap between game and announcement.