Because bruisers, in general, are useless when not strong. It's not even Riot's fault. It's part of their inherent design and the inherent design of League.
As Chipndip said: You only have 5 people. One has to be a marksman because nobody else can be better at taking turrets. You need a support because marksmen aren't able to exist without one. You need AP damage, which should be a third slot. That leaves you two slots, and 2-3 people that need to be peeled for. (3 with enchanter, 2 with tank support). You want at least one good person for peel, and you need someone for engage. Neither of which juggernauts can really do. Divers can engage, but they also need help to do. Tanks can peel and engage.
2 tanks is almost always going to be your best strategy for the other two.
The first reason I said is because of inherent design. Brusiers as a class are fairly general. They're not good at doing anything in particular. DPS? Marksmen do it better. Burst? Mages do it easier, assassins do it better. Tank? Tanks are better at it. Engage/disengage? Tanks. CC? Almost anyone else does it better.
As the current definition of bruiser is, and with the current map (5v5), they just won't be useful unless they're too strong. We'd need to have a need for what amounts to a generalist class first.