The reason why Janna had a high win-rate for seasons is because she promotes a playstyle that is inherently more successful in solo queue

Janna has always had a high win-rate, as a recent frontpage thread mentioned, which is true. There were even phases where she benefited a shit ton from the meta, like the season 7 Ardent Censer enchanter support meta. But in general, she has always had a high win-rate regardless of meta (with a rather high pick-rate). Now, what's the real reason her win-rate has been and is that high?

Imo, it's pretty straight forward.

Janna is the epitome of a safe champion. She forces you to play calmly, passively. Please, read this out - I'm not talking about her current laning phase, I'm well aware of the lvl 1 W making her a lane bully in some matchups. I'm also not talking about the "just stand behind and press E" meme, a good Janna offers much more than this.

Outside of lane (where most games are won or lost), she is not rewarded for cocky plays, but rather for cock-blocking (well so much for the memes). With a Janna, you won't promote the 1v5 plays your average solo queue Vayne does, but rather prevent it (contrary to like, e.g. Leona). Everyone stuck in lower ELO would actually get higher up in solo queue if they implemented the gameplay that is most inherent to Janna: Don't go all in, prevent your death, farm up, play around objectives. There is a reason why we tell new players that they should focus on CS'ing - it's the first "objective" you're going for when the game starts. Going 0-0-4 is oftentimes better for your solo queue success than going 4-3-0. Killing your opponents while at the same time dying is less of a step towards winning the game than just taking a dragon or a turret without casualties. Solo queue is not about killing your opponents by outplays, but outsmarting them. Chances are if you have to fight (and die) for a play, you could have gotten more out of it if you just survived (with, let's say, the enemy laner or team backing off). Once you get higher up in the ladder and start reading the game, that's when you should use a more offensive playstyle to create and push advantages.

This is coming from a hardcore playmaking/tank support player. I'm definitely not defending her.