Riot, this hurts.
http://current.lolalytics.com/champion/Diana/Middle/ https://na.op.gg/champion/diana/statistics/mid
Greetings, I am Kammavipaka (NA), I was formerly Full Moon Ace (NA) and even further back was RefurbishedIce (NA). I have been an avid Diana enthusiast since the fall of 2015 and own every skin and almost all the chromas for her.
Since the middle of 2016, Diana in 2 roles has made a steady decline in terms of overall power after the meta shifted away from Marksman, repeated nerfs to her itemization (most notably Abyssal Scepter), meta shifts throughout the midlane and jungle, the reworked jungle in Season 7 have made Diana a much harder champion to play, and even harder to enjoy.
6.22 (preseason) and 7.9 (Midseason) patches were really hard on her. Pretty much all of the match-ups that she excelled in, most notably Fizz, Talon, LeBlanc, and Katarina were reworked in a way that made it harder for her to survive those matchups. Abyssal Scepter was infinitely better on Diana than Banshee's Veil ever was or will be, not to mention the passive on Banshee's Veil windows Diana's power exceptionally hard as it denies a reset on Lunar Rush.
I vaguely recall Ghostcrawler in a Video Game Designer's event saying something about how some champions needed to be shafted due to an extremely poorly designed kit (such as Azir or Kalista) and how it hurts developers and players who develop an attachment to that champion. Even though Diana's kit feels quite binary at lower elos, especially Bronze and Silver. She still has sufficient counter-play and is not even remotely to balance for competetive and solo Q. Heck, she can be viable in Solo Q and still not be picked in competitive.
tl;dr why is Diana in the gutter.