So, why cass has that bad stats?

Because she's one of the worst champions in the game and buggy as fuck. I used to main her in Diamond, but she's unplayable right now.

I mean, unless you like being shit on by every burst caster and champion with mobility, having your ult be extremely unreliable (and I don't mean because it's telegraphed), not having enough mana to kill anyone in the early/early-mid game, and having your primary damage ability randomly bug out. In which case she's great.

Because the ChampUp team decided a few months ago that working other champions (Fiora) was more important than working on fixing what they previously messed up (Cassiopeia).

She is difficult to master and has very subtle windows of opportunities. A very small misstep can mean the death in both the laning phase and teamfights and this goes both ways (Cassio or her opponents).

You really need to know her damage vs mana costs at each stage of the game and be able to know when you will be able to kill/chase somebody or when you will go OOM, many people that learn to play Cassio see the dmg at lvl 2-3 and think they can all-in but then they run OOM before they kill their opponent. So they type "/all OMG ur so lucky", no, they miscalculated.

Missing just ONE Twin Fang execute on a minion can make you lose your laning phase, that's really harsh and it really is this missed minion at lvl 2 that can get you killed/miss a kill at lvl 4.

She has a very varied item pool, but she also needs this variation. People that think they can get away with one single item build every game (Which does work for 75% of mages) will have a hard time.

TL;DR her win vs loss cases are unclear (mana management, positioning, itemization); this leaves people wanting to learn her frustrated and they dont pick her up.

Try playing her in Diamond+ then tell me you still think she's good.

The nerfs hit her exactly where it hurts her the most, her (lack of) sustain. Her manacosts already are horrendously high and her lanephase is extremely unforgiving, it's quite easy to snowball against her if she gives first blood. Her winrate is far worse than what it was before the rework whereas her pickrate went back to what it was in 4.16. She has gone full cycle and has returned to the same pitiful state she was in when the initial rework happened.

Riot has yet to understand that she's unplayable if she can't even properly retaliate against the enemy midlaner if she goes oom instantly while they get away with enough hp to bully her out of lane. Her range is too short, her mobility is too unreliable and her squishyness makes any damage she may acquire entirely pointless since she dies as quickly as her opponents. She's the worst "hypercarry" in the game right now.

Cassiopeia has a lot of damage potential, but her playstyle makes it too much of a high risk to pick her when you can pick a or a without worrying too much about your performance, really, some stupid clueless people just say "Oh cass, QEEEEEE" it's not like that, in TF you have to have the skills to keep poisoning people, not to mention her poor target switching ability. Her best and almost only damage source is her E, put it in cd and you are nothing, if you miss click on a unpoisoned target in a TF, then you are just a sitting duck. She is too much of a risk for not that much of a reward, they just kept nerfing her bad, she is not really that rewarding to play anymore. But our words have meant nothing for over a year, Cass never forget..

right away on that graph "win rate by game length"

it only curves up.

its because her passive and her abilities scale into late game, her somewhat weak earlygame just gets picked on by junglers/support ganks, etc etc, you cheese her and you get big off of her because she has no built in escape.

but at the end of the day, shes a monster as you get later into the game. its the same reason people dont play as much twitch/veigar/anivia, who need items to be really effective, instead players prefer champions who have useful abilities, like ahri, janna, irelia, hecarim etc etc