Ok...*in the states*...
Because, as far as I know (indeed, as far as I'm concerned) no one paid attention to any of those bands (and really, Venom was probably the first) until KEA and RTL started making waves. Then suddenly, everyone was checking out the whole scene.
Heck, I had been a metal head for 4 years, and I was getting sick of how Hollywood-ized everything was already starting to become. I saw RtL on the "New Release" rack at my local record store, and it wasn't until then that I began hearing about Slayer, Anthrax, Venom, Kreator....etc..etc..etc. And at my entire high school, I was one of two people that had even heard of Metallica...much less the others.
I guess I'm digressing. But my point was that they were part of a scene where (to the typical 14 yr old boy like myself) the heaviness and speed was what set them apart from the mainstream metal bands of the time (i.e. Ozzy, Motley Crue, Judas Priest, Iron Maiden, and the "just starting to rise" glam scene)