Darius is a champion that just kinda has a lot of bullshit going for him.
First things first, darius is just one of those champions where you win or lose in champ select. Most melee top laners lose super hard to him, and even in the very small pool of melee champions that do win against him, like kled or renekton, the skill required to do so is still plenty more than the amount required to do as well on darius.
If you're second pick top lane, and he's last pick? Sorry dude, if you picked a champion that doesn't kick the crap out of him, you're probably going to lose. Combine this with the fact that he's very easy to pick up and do well with relative to a lot of other top laners, which means he tends to be a very common pick, and you have a pretty decent choice for a ban.
He's got the uber oppressive early game of pantheon, without making the tradeoff of a god awful late game that panth, and to a smaller extent renekton, have to deal with, which means that not only will you ever actually truly put him out of the game, there's never really a point where he's weak. He's strong pretty much always, and his only real limiting factor is how well the enemy team can kite him, which in part is removed from the equation by
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In lane he sustains out the ass from his Q, forces you to give up lots of farm in order to not die (provided you're melee), can easily 1v2 past level 6, wins every 1v1 at level 1 (once again, if melee), is really good for ganks due to his slows and his pull, can kill you if you escape/ flash out with small amounts of health with his bs bleed, can fully proc the armor shred on cleaver from 1 AA and his passive, and gets insane amounts of extra AD from his 5 stack passive, which also feeds back into the damage on his bleed.
On top of that it isn't actually possible to effectively itemize against him. Rush armor? His true damage ult is gonna take advantage of the fact that you're not building extra health. Rush Health? The armor pen from E is gonna reduce what little you have, and with no extra resistances the damage is gonna hurt quite a bit. Rush damage? You better pray to god you can 100-0 him in one rotation, because you likely wont survive past that. It's not possible to effectively itemize against his damage while still building damage, which is what you'll need to actually try and fight him.
It doesn't help that he's very flexible in what you take for summoner spells and masteries,
are all pretty viable picks on him, and he can make very good use out of grasp, fervor, and stormraiders, so he's very versatile in how you build his masteries.
I myself, as I've been spamming top lane
for a while now, tend to either ban renekton or darius. While renekton just kinda shits on kled, darius is more of a skill matchup, but its not freelo like some people say it is, and while I may stomp on a darius who doesn't know what he's doing, if im going up against a more experienced darius, it could easily go either way.
Is he the most BS thing on the planet? Probably not. But darius just kinda hits that glorious tri-fecta of oppressive ass early game, very low skill floor (which tends to result in high play rates), and countering a massive amount of the more common top lane champions, as well just abusing the crap out of a lot of smaller things, like his interactions with cleaver, that tends to put a gigantic target on his back for bans.
While there are lot of champions that are also targets for bans, like say, fiora, twitch, and blitz, in many cases its only a couple things about them that tend to be the issue, such as twitch's AOE burst, or blitz's hooks, whereas with darius, kinda everything makes him banworthy.