If he gets on carries, he sticks on them just fine. It is getting on them to begin with that is the problem and should always remain the problem because that is his counterplay.
Champion.GG is known to not be up to date and flushes their data a lot.
http://na.op.gg/statistics/champion/
This is a month sample size
Tanks with a 50 percent or better win rate over the last month
- Singed - 54.31%
- Sion - 53.63%
- Rammus - Im throwing him on here since in Diamond he is pick or ban in every game so Darius would see him a lot - 52.47%
- Mao'Kai - 52%
- Garen - 51.38%
- Nasus 51.37%
- Malphite 51.36%
- Illaoi (I consider her a Tank, some might not, it depends) - 51.32%
- Galio - 50.81%
- Cho'Gath - 50.35 %
- Dr. Mundo - 50.03%
This is all Plat and above stats which is where Darius has struggled. Tanks are picked the most top and while he is good at shredding tanks, the itemization and runes have made it so tanks can ignore him until he reaches his first full item.
This buff allows him to have access to early armor pen faster then the enemy can stack it as well as his main trading tool on a lower cooldown allowing for two W's in a full on trade.
He is not THE definitive counter to Tanks being picked top and that is what Riot is going for.
If he could also be unkitable he would just be broken as hell