There's a lot less dignity in metalcore and deathcore. It's fun music for sure, but it's so much hokier than death, black, thrash, doom, trad. I'm sure somebody will just say "lawl metal is teh joke alreadyz" but I don't see it that way. Sue me, I guess. I still spin some deathcore. It's fun, but it just doesn't have the weight and substance of other forms of metal. Is the weight and substance of death, black, thrash, doom, trad all in my head? Sure, maybe, but in what sense would that change my opinion on deathcore and metalcore?
Plus they just seem a lot more bland and samey, in general. There are bands of these genres I like, but for the most part I find them much less inventive and enjoyable. There's also the fact that metalcore made its rise in the 00s, the era of over-production. I dislike slicked-back modern DM and BM almost as much as I dislike your average slicked-back deathcore and metalcore band. Also there's the issue of the vocals. It's like they decided to exchange demonic, evil, inhumane vocals for whiny whine-whine vocals. Why they made that decision, I cannot comprehend.