We'll get into that in some upcoming articles!
Basically, if it's flat then short-mid range ADCs get way more power overall. Using % based range is both more fun to find crazy combinations (Kog'Maw W + Milio W) and makes Milio have preferred friends. We had concerns about him being too good with everyone.
Knowing you have to deal with longer range from long range ADCs is annoying, but they have kits built for long range champs. Turning short range champs into long range champs is terrifying because their kits are powerful and range is a normal limiter.
Using % increases allow that dynamic to stay in an intuitive way without losing those exciting combos and extreme situations that are countered in the same ways as before (dive the backline).