I don't think it has anything to to with, interviews, warming up, lazyness etc etc.

if you look back old performances , mariah really gave her all.. just like whitney.. Through the years I think they just damaged their vocal cords.. And i dont know if it can be fixed... But they're not the only ones.. did you see what happened to Usher last friday? and also look at Kelly Clarkson, her voice started having trouble before her 3rd album came out..and dont forget Lauryn Hill

I think that all those amazing vocals start having trouble after performing so many years..

It's a shame because they still have the drive to keep performing and make great music.

And in Mariah's case I dont think they want to admit that the voice isnt there completely anymore, because they would definitely lose sales. But I think that if she would just admmit it , it wouldnt be that bad... and just do some live performances once in a while..

I dont even understand why se decided to sing live in Japan, she almost never sings live abroad.