I dunno, it just has a certain je ne sais quoi. I'm sure my love of this album is partly due to when I heard it, but I don't think it's just that (there was a lot of electronic music I got into at the time but most of it hasn't managed to maintain such a special place in my heart). But I think you're doing it a disservice by comparing it to the likes of Solar Fields, Aes Dana or Asura (I'm not too familiar with the other two artists you mention); they make psybient, whereas Amber is much more of a "pure" ambient album. It's not something that can be enjoyed in a tent while sipping chai and discussing the military industrial complex with a man with dreadlocks, it's something that I listen to alone in the dark when I'm in just the right mood, and when I do so it messes with my head in a way that most other music can't.
Also DAT COVER