Because mobs get progressively harder. Think of it as the Cataclysm content, where difficulty started spiking later on.
Either way, they just need to tweak things like Chi Wave prioritizing you for healing bounces,etc. And they need to change that useless dodge % artifact talent into a flat % damage taken decrease just like other classes have. Also doesn't help our artifact healing is tied to burning our transcendence cooldown instead of on some more natural spell that doesn't disrupt rotation/placement as much.
I also want my Zen Meditation back ;(