People will come to the CoHP, but it still can't hurt to even provide even something like a section of the Expo where it hypes up the new movie and shows off certain props, locations, etc that haven't been announced yet.
And yeah, the POTTER brand is huge, but the Fantastic Beasts brand is somewhat meager. It had a tepid response in 2016 despite making over $800M WW. Mostly people just went to see it because they were Potter fans and many came away unsatisfied. And there's already controversy going into FB2 about Johnny Depp playing Gindelwald. You'd think they'd try to get fans excited for it, even if it is 10 months away still.