Bruisers are stronger in lane than every champion type except themselves. They have higher base stats, better and more consistent trading ability, and so forth, part of the reason they can usually jungle but AP mages can't. The only reasons we put AP casters mid is because they had CC and mobility that allowed them to gank and to a lesser extent because a balanced team needed varying types of damage. But bruisers now have so much control and mobility, and with the inclusion of Black Cleaver and the ArPen changes, magic damage isn't super necessary either.
Recently, all AP are bringing to a team that can't be offered by someone else is magic damage, and frankly, that's not good enough. Bruisers and AD casters have as much if not more burst, as much control, and in many cases, more mobility.
AP casters are losing their place in League of Legends. They're becoming a frail, immobile liability. They don't scale as well into late game. Magic damage isn't enough. Soon, many if not most of them will cease to be used. It's already happening and it's really disappointing.