It certainly isn't - it's because she was never as immensely popular as Adele is right now. Simples.
No, it was because for the last few years of her life, Amy was a hopeless addict and then died at 27. She never got the chance to make it in the US like Adele has, drink and drugs saw to that.
Who knows what would have happened if Amy had cleaned up her act? We'll never know will we?