The recent tank changes and how they effect an already bad matchup
With the recent changes to tank items lowering HP in favor of armor/mr, there is no way to itemize against Darius to keep you alive until and through the mid-late game.
only have 425 HP now. Only 25 more than Cleaver, while their components grant less HP until completion.
has only 350 HP and is aimed far more towards reducing critical strike damage.
Mallet and Warmog's grant no armor, so other physical abilities will do more damage to you if you attempt to buy HP to counter his ultimate. They're also not very good in terms of items to rush in the first place.
Meanwhile Cleaver, which he rushes first, is now 400 HP, and along with his E makes him ignore 30-50% of the target's armor. (and with the time between an E and a Q, cleaver will be fully stacked from his passive). That's on top of the fact Darius has more base armor than every other champion in the game besides Rammus.
What this amounts to is a continuation of the trend we've seen since his release: If you counter-picked him, it's too easy to win. If you didn't, it's too easy for him to win. There's no counter play at all if the match-up isn't already in your favor. And now the advantage he already had against tanks is far greater due to their new added vulnerability to his ultimate and cleaver.
There has not been a single patch since the juggernaut update where Darius wasn't within the top 12 bans. And the explanation I gave here tells us why he suddenly hopped back up to the 5th spot.
Plenty of people like to play him because the match-up is favorable more often than not. And even if the matchup is unfavorable, it's far easier to play safe and farm with his abilities than it is for anyone who's forced to do so against him. But he fits the term "lane bully" far better than any other top laner. Even some others who are able to win against him can only do so because they directly counter his kit (like Pantheon's shield to block early hemorrhage stacks). Ask any high level player and they will have nothing but counter-picks as advice to beat him, because even if you try to stay safe under tower he's still guaranteed to beat you in farm by several kills' worth of gold if he's not stupid enough to Q minion waves needlessly.
He needs a true rework. In the past, they've reworked feast-or-famine champions because they needed a lead to be unstoppable. But Darius is a case where he's definitely feast or famine, but the condition is not early performance, it's simply the match-up.