It's likely they're purposely slow-building this attraction because they want to be able to open up something big every year. New Illuminations in 2019. Rat in 2020, GotG in 2021. And so, it's likely they started much earlier than they needed to because Ellen's was falling apart and no longer drawing crowds. And so, this means quickly build a shell, then take their time with everything else (especially since it'll be a year or so before they can get the cast conveniently to file the pre-show elements).
It's almost like WDW just recently realized what opening a new attraction on a regular basis can do to attendance (and revenue).