I don’t really know that it /is/ necessarily ‘harder’, but the focus is shifted and that’s why I don’t tank that much any more.
Tanking used to be primarily about three things: gear, threat, and positioning.
You weren’t tanking anything but the floor without enough gear to let you survive (read: Defense “soft cap” to avoid crits; armour/block rating/value to smooth out incoming damage; stamina to absorb big hits and give heals time to land).
You also weren’t tanking anything if you couldn’t hold threat. You had to actually be actively pushing your buttons - not to do damage yourself - but to give your DPS the threat threshold to do their job (read: damage). The more threat you could put out, the more damage your DPS could do, the faster you were done the dungeon. But DPS had to be mindful, too. Be on the right target, watch their procs and stop damage if they started “popping off” ::cough Windfury cough::. Threat management was a two-way street. It was an aspect of teamwork.
Positioning was a bit less important, but on some fights it was absolutely vital, and in most fights it can help a lot (especially with melee DPS). Knockbacks, conic attacks, moving bosses so the rear arc is out of the “fire”.
These days tanking seems more about matching your “active defense” to the boss’ attacks and not a whole lot else. Harder when you don’t have timers. Pretty trivial with timers.
Gear is whatever, sure you can optimize it, but you’re given crit immunity for free, and everything has oodles of stamina on it. Threat is whatever, unless it’s the very start of the fight, or you go AFK for 5 minutes, no one’s pulling threat off a tank. Positioning seems to be largely ignored unless the fight absolutely demands it, and even then, I can’t tell you how many times tanks force me to attack the front arc because they keep the rear in fire.
So yeah. Forget tanking in Retail.