April/May because of several reasons
>game already announced
>launch window already revealed
>Masuda admitting announcing so early with SM was a mistake and saying things would be done differently in future
>Nintendo historically don't like talking in detail about upcoming releases until they're about 6 months out from release
These all combine to ensure that an early reveal is unlikely, as we've gotten most of the pertinent info a Jan/Feb announce trailer gives out (announcing a new Gen game for release later in the year). It's not vital we get the titles now and last time they did an early reveal, we didn't even get a proper look at a new pokémon so there's that as well. A reveal later in the year however allows them to be able to show off more first time out (they know this thing's eagerly anticipated, the noises surrounding it both officially and leaked indicate they're aiming significantly higher than past editions) to go in-depth a little more and to make first impressions count more, rather than risk repeating Gen VII's announce/reveal fiasco.
