Some monsters abilities are hard for some heroes to deal, for example, "fast" if you are playing a Wizard.

But when you put stuff like "Fast", "Teleporter" and "Vortex" all together, you condition players to just start skipping monsters. It's like every other mob is a "triple shot, lighting enchanted" elite. You could, theorically, kill theses mobs. In pratice, you lose way too much time to be worth.

A friend said Blizzard did it on purpose. if they did, they had a big brain fart on the brainstorm room, or just wanted a lazy way to make hard content. It's hard, yes, but hardly fun.