Bruisers tend to lack the crowd control or other abilities which makes the entire enemy team want to focus them. They do, to some extend, have survivability, but most tanks also have abilities which increase their durability.
The gap between the two has grown more narrow though. Take Nautilus or Amumu - if you ignore them, they'll CC your entire team. Similarly, Shen, Galio or Rammus will just taunt you and render you useless for a while. Trundle or Shyvana on the other hand are sturdy, but have nothing to force you to focus on them.