If I had to guess it's got something to do with what happens every time Riot looks at item cost.
Tank items get more expensive while damage items get cheaper, this when combined with this season's masteries means that 'tanks' are squishier than they have ever been in the past. Used to be that a tank who'd gone full defense had enough hit points that you couldn't kill him first, no matter how many people on the team attacked the tank he'd last at least 10 seconds which was more than enough time for their team to clean up. Now tanks die in 2-3 seconds at the start of a fight, before people even bother with the carries. It's all kinds of screwed up.
Just my point of view, I do realize that it's not a popular opinion.