There just wasn't evidence-based studies for a lot of common dental practices. Not unlike when the FDA was created, a lot of old medicines were just given a pass, even though they aren't useful (like how Acetaminophen is barely better than placebo — it'd never be approved today).
Just because there's no actual studies for flossing, that doesn't mean that flossing is bad or not-needed per se, but there does need to be more basic-level studies for it. I had bad gum-disease in my 20s, but once I actually started flossing daily, it stopped progressing. So it clearly helped me, but a better study on whether everyone needs to floss and how often should be done.
Just because there's no actual studies for flossing, that doesn't mean that flossing is bad or not-needed per se, but there does need to be more basic-level studies for it. I had bad gum-disease in my 20s, but once I actually started flossing daily, it stopped progressing. So it clearly helped me, but a better study on whether everyone needs to floss and how often should be done.