Not a paleontologist, but I think it’s a mix of both wrong information being spread back then and also new info being discovered.
I’m pretty sure people knew that birds were dinosaurs for a while, but people just liked the idea that dinosaurs were monstrous lizards. Giant monsters just capture the imagination in a way that giant birds can’t.
And then paleontologists started finding fossils that had imprints of feathers still on the body, and it became really hard to ignore that dinosaurs were a lot more bird-like than people would like to believe.
My impression has generally been that once dinosaurs started to be viewed as bird-like, people started to see them as animals rather than as monsters, and that just kinda snowballed into dinosaurs becoming more and more bird-like