I think the rise of metalcore is largely a reaction to nu metal.
Metalcore borrows a lot from the thrash and heavy metal bands of old like Testament, Metallica, Iron Maiden, Slayer, Anthrax, Judas Priest, Pantera, Biohazard, Sepultura and bands like that, mixed with the aggression of hardcore.
Some metalcore bands that I can think of are Killswitch Engage, Shadows Fall, Lamb Of God, God Forbid, Trivium, Chimaira, Bleeding Through, Unearth, As I Lay Dying, Darkest Hour and a whole slew of others.
I think that, nu metal as a genre has ran out it's course and has become diluted and some musicians and fans crave for something more pure and closer to the metal side of things. But now things have splintered off to different sub genres like screamo, emo core, math core etc.. that sometimes the boundaries of these sub genres are kind of blurred. Is it a phase? Who knows. The thing is, from thrash metal to grunge to nu metal, once a certain genre becomes blown up and huge, there is always a danger of it becoming saturated and diluted to the point that it will become redundant, until a new musical movement comes in.