That's because Amy was plagued by alcohol and drug addiction, she wasn't strong enough to cope with success. She had no self confidence at all, poor girl was tortured by her demons.
A legend like Tony Bennett thought she was something special, and he would know.
What? I did say she was brilliant but again this is nonsense when comparing to Adele. '21' must be one of the least personally-promoted best selling albums in history (the woman isn't even seen anywhere for months and months on end let alone witnessed promoting the album). But, like I say, if it makes you feel better to think Adele's success is mostly down to Amy's passing then go ahead. Just don't be surprised when this 'theory' is shown up for the nonsense it is.