You're asking the wrong question. The real question is "Why would you ever play bruisers when tanks exist." Because Tanks usually end up out-classing bruisers in every way. They don't die as quickly as bruisers, they usually have more effective cc than bruisers, and their base damages make them less dependent on items. Most of the time, they can outlast bruisers by just soaking up damage longer than it takes to kill the bruiser. Seriously, the tank items we have right now are really strong.
I think assassins are kind of a bit of an upswing right now, which I personally hate because my ban that I usually reserve for Xerath because he's so annoying is now taken up by Zed, but bruisers can still abuse some of these tank items. Titanic hydra in particular can give bruisers added tankiness and wave clear that they really shouldn't have, and Dead Man's Plate gives them better engage than they have any right to have. It depends on the case, of course, but Bruisers have some advantages that assassins don't. Assassins have some advantages that bruisers don't.