it's pretty simple actually, bands like zeppelin and sabbath gave way to bands lke maiden and priest, which, when combined with the speed and aggression of punk gave way to thrash, it got faster and faster, becoming grind. While that was happening, bands that took the heavier is better approach to thrash, bands like death, celtic frost, hell hammer, possessed, etc. all wound up stumbling on early death metal. from death metal evolved black metal. Metalcore and nu metal, etc are just attempts at commercialization by the addition of more palatable elements, and or a fundamental simplification of the formula.