Simon, you have to consider this though....
When Helloween first "broke" in the US was with Keepers I and II.
Sure, some of us in the underground knew them from the Judas EP and Walls of Jericho.
This was at the time though when the underground was getting even heavier, with bands like Frost, Kreator, VoiVod, etc.
Helloween were always stuck in the middle, because even though they came up as a speed metal band, they were getting a bit to "cartoonish" for the thrash crowd.
After Keepers II, their albums were no longer available in the US.
I seriously didn't know until almost 10 years later that there were two more albums with Kiske (in Pink Bubbles and Chameleon).
When Castle started to distribute in the US, that's when you would start seeing the Deris CDs floating around.
At that point, Helloween here in the States were just "another classic band with a new lineup"
It's the same reason why bands like Metal Church, Candlemass, etc, etc draw very minimally here in the States these days as well. There was a HUGE lull in their album's availability here in the States.