Movie-goers already know what happened to The Losers Club members as kids, so the sequel doesn’t have to re-cover all that ground.
The book can get away with flashbacks as resurrected memories, but that’s because first time readers aren’t aware of the story.
Andres Muschietti has confirmed that IT: Chapter 2 will feature flashbacks and the return of the young versions of original’s central cast, and this is a great thing… provided that the movie doesn’t lean on them to heavily.
Instead, the next film would be much better suited creating a different narrative device that recovers memory, and instead use the flashbacks to showcase and create character moments that we haven’t already seen.