I don't think tanks are inherently less tanky atm, but they opt out of tank runes to get early power
They'll avoid aftershock in favor of runes better for laning and skirmishing (e.g: comet in lane on cho, fleet in jg on sejuani), which makes them squishier. If they took the designated tank options, they'd probably do fine at tanking.
A large part of the issue is that unlike the old bottom-up mastery system, where your weaker mastery choices locked you into a keystone, the new rune system is top-down: you don't need to invest 20 mastery points in a bad tree for a tank before you can access comet. Furthermore, you lose a lot less going resolve secondary than you did going 21 offense/9 defense, since investing in more offense oriented runes isn't nearly as bad as investing in the offense tree used to be. You give up a lot less in the way of tank stats than you used to. (Hell, sometimes you still get really nice tank stats from non-Resolve trees -- Legend: Tenacity, anyone?) As a result, tanks are less tanky because they can opt into early power without totally crippling themselves. They're still way worse at being tanks, but they're less abusable in lane. This means they don't scale half as well as tanks as intended.
Arguably, another part of the problem is how much of a tank's tankiness is frontloaded into aftershock. Instead of a smooth tanking curve, you're unkillable for 2.5 seconds and then your ability to tank drops off like a rock. It also makes health significantly more valuable and lowers the relative value of armor and MR, skewing tank item purchases and power.
tl;dr: runes are fucking up how tanky a tank is because tanks are opting out of tankiness, and aftershock frontloads tankiness