It mostly gets hated because it's a genre. Just a lot of arbitrary title-making and drawing lines in the sand (regardless of how close the tide is). That's really all that matters, but I'll elaborate cuz it's late, I'm online, and I love talking about music.
The reproductive cycle of a genre consists of three major stages: The Innovator, The Propeller, and The Glut.
The Innovator is a band or artist that evolves out of a prior scene and brings some new, radical ideas to the table. It doesn't belong to a specific genre (at least at first) and often spends most, if not all, of it's career in the underground. It takes what's been done before and breathes new life into it, but sometimes has trouble connecting to a mass audience. In the special case of Metal-Core, most of the credit for this goes to the Gothenberg scene, like At The Gates, Dark Tranquility, or In Flames (who extended their career by crossing over into Metal-Core, to the chagrin of many). Credit also may go to some early screamo acts, like Refused or At The Drive-In.
(it must be noted that the innovator does not always lead to a genre, it may just end up as a totally unique band that lives on in it's own little bubble, which is fine)
The Propeller is the band who catches wind of the new movement, started by the innovator, and ignites it. This band has enough creative juices to compete with the innovator, but shines even more in it's ability to deliver it with more energy and mass appeal than it's predecessor. It at once cements the new bonafied genre and places itself atop of the hill. It also ends up being the most divisive band, both loved by people excited about the new blooming genre, and hated passionately by people annoyed by the new looming genre. So now we're talking about Avenged Sevenfold, As I Lay Dying, Trivium, Killswitch Engage, etc...
(the media's role in the love/hate shoulde noted, as the propeller is the band that magazines/webites will rave about on the upswing, but start to deride once they feel the need to appear hip and forseeing about whatever new thing is on the horizon)
The Glut is the massive shit taken by the previous two, the hive swarm of sound-alike, copycat, no-talent bands that flood the market, poison the well, beat the horse and fuck the corpse. They don't add to the reproductive cycle, they gangrene and fall off, but not before sucking in as much hate for the genre as possible. The propellers, being legitimately talented (though divisive), end up getting shit on in the process by critics who can't tell the difference. There is something that can be said about musical taste being subjective or objective, and if you like or dislike a propeller, it's really a matter of subjective taste; liking or disliking the glut is a matter of objective intelligence. I'm not going to name bands for examples, because fuck them.
But here's where things get important, and where you really have to be on the lookout. While the glut is writhing, festering, and fornicating with itself, out of it's very bowels may come a band who looks like all the rest, and in it's inexperience may even sound a little like the rest (at first) - but just a little off. Just a little different. Just a little better. You may be witnessing the birth of something new. Something shiny. Something as quiet as whisper which will echo into the valleys eternally until it has mustered an unholy avalanche. You may swear off a genre or a fashion or a scene, but when it emerges and stands utterly alone, sillhouetted against the shit-tornado that obscures so much of your vision, and spews fiery liquid gold at your face, you will know it's name is The Lord when it lays it's vengeance upon thee...
(lyrics relevant, actually)