There seem to be a whole series of reasons.
Lizzie said "jealousy", and she is right, but part of it seems to be a feeling that the artists in question are sucessful, but don't deserve to be. Perhaps because their music is simple and easily accessible to the general public, the critic feels that this is a result of a lack of talent or seriousness. The way in which "popular" performers like Daniel O Donnell or Kenny G are reviled typify this motivation.
As the Villan says, far too many people are unable to make the disinction between "I don't like it" and "It's bad music".
There is great music I don't like (Mahler or Nine Inch Nails for example) and rubbish that I love (Jonathan Richman or Old Rock and Roll Ballads).
Then there is the underlying idea that only a good person can make good music. If you follow this line of reasoning, you despise the music of people whose lifestyle or opinions you disapprove of.
The truth is, you can make excellent music with people you can barely bear to speak to, and people whose views you despise can blow you away with their music.
Cheers
Dave