Because it's the only way you're going to fix things.
If you:
Then you should get flagged as throwing a ranked match. After three of these you should get a 10-game Ranked Ban with no long-term consequences after the fact. If you don't do this, ranked will never be cleaned up.
KoreanEzMode1 votesOh i thought this was gonna be a legit reason post, but nvm just another beautiful toxic member of this community raging on forums
RookPusher1 votesOh i thought this was gonna be a legit reason post, but nvm just another beautiful toxic member of this community raging on forums
It is legit. You assuming that this isn't serious is toxic. Explain to me how this isn't a way to clean-up ranked without letting the policy run wild?
No Excuse1 votesYikes. Where do I start? LOL. The only reason imo someone should be banned from ranked is intentional trolling/running it down mid, and excessive toxicity in chat and harassment (chat ban maybe?). Your criteria is both hypocritical and nonsensical. Addressing your points at a time:
(1) Many tank/diver players have won many games through game-deciding engages resulting in our deaths for an ace. 
(2) I consistently play many different champions and styles and perform considerably well. I play ranked as a "fill player" and I still have above a 55% win rate. Should I get banned?
(3) lul? Not an argument but a conclusion.
I'm going to guess this is just a rage post though. Looking at your match history, you have only 1 champion where you've played over 50 games. I.e. you'd get banned from your own policy??? [sg-lux-2]
RookPusher1 votes[{quoted}](name=i wiIl ƒk u,realm=NA,application-id=3ErqAdtq,discussion-id=j2EGn5EU,comment-id=0001,timestamp=2018-06-15T17:22:38.532+0000)
Yikes. Where do I start? LOL. The only reason imo someone should be banned from ranked is intentional trolling/running it down mid, and excessive toxicity in chat and harassment (chat ban maybe?). Your criteria is both hypocritical and nonsensical. Addressing your points at a time:
(1) Many tank/diver players have won many games through game-deciding engages resulting in our deaths for an ace.
(2) I consistently play many different champions and styles and perform considerably well. I play ranked as a "fill player" and I still have above a 55% win rate. Should I get banned?
(3) lul? Not an argument but a conclusion.
I'm going to guess this is just a rage post though. Looking at your match history, you have only 1 champion where you've played over 50 games. I.e. you'd get banned from your own policy??? [sg-lux-2]
It's not. And I'm going to guess you're new to debating.
If those tanks routinely dive, then the average will be higher. This entire point was to provide breathing room for tanks. Assuming you're not outside of the average + 5 you're not subject to this ban (this is an example). If the average Jarvan has 7 deaths, and you have less than 13, you aren't subject to this ruling.
Again, there are criteria built in here so you're not unduly subjected to this rule. If you're that good, then you're not giving up an absurd amount of deaths. So then this ranked ban isn't subject to you. But the larger point here is that anyone I've ever read has suggested the way to climb is by playing the same champs consistently. Even if Riot doesn't ordain it, it's statistically accurate that small pool players win more than large pool players.
Therefore, the inverse, which is playing multiple champs constantly is the way to NOT climb. And in ranked that should be seen as trolling.
Overall, the thing to keep in mind here is that this isn't a perfect solution designed to catch everyone, its designed to shave off the obvious trolls, restrict their options to troll, and for those who just can't figure it out, a break that reminds them you can't (in ranked) just do whatever the hell you want.