Why didnt metallica follow the thrash metal code ,thrash till death?

Because Kirk and Lars suck. Kill Em All, Ride The Lightning, Master Of Puppets and ...And Justice For All are some of the best thrash albums of all time but starting with the Black Album they went more to the mainstream. Death Magnetic is arguably their best album since AJFA though, its at least sort of thrashy. Most thrash bands went through similar but shorter stages of that too. Anthrax was **** the entire Bush era, Megadeth releaed Cryptic Writings and Risk, 2 horrible excuses for albums, and Testament went mainstream with the Ritual, which was shunned by pretty much everyone