I honestly think that it's because of season 4. The infamous tank meta, where tanks were so effective that it was common to see many tanks in 1 game, and it was common to build full tank on assassins and still one shot squishies. I think riot doesn't want to recreate that, and as such, keep damage somewhat high, so tanks melt quite fast.
...However, its an idea thats only good on paper, extremely poor in execution: Damage is so high it punishes anyone who gets caught out equally, be it tanks or adcs. We've gone to the polar opposite of the spectrum, where damage is high and tanks are low. There are other questionable "nerfs", with the kaisa "nerf", the riven "we nerfed her too hard so heres a buff", and kicking poor aatrox down into the dirt. It's kinda clear that damage is kept high/adjusted specifically for what is fun to see in the LCS, and while I don't blame them for wanting the money from skins or tickets, it is a bit worrisome about the future of this game.
I hope a middle ground is somehow found, between "why is 300 armor talon one shotting me" and "why is maokai one shotting me" :/