The reason is that Celtic Frost, in their album Into The Pandemonium, had experimented with different influences on Thrash Metal base at the time no other band can dare to. Frost is the basis of '90s metal genres; Gothic Metal,Doom/Death Metal,Funeral Doom Metal,Symphonic Metal etc... Now the genre Avant-Garde Metal is used for the bands, who have progressive approach but processed differently from traditional Prog-Metal acts, adding more varied and non-metal influences to their music. Like The Sham Mirrors-era Arcturus or Kayo Dot or Jazz-influenced era of Ephel Duath.