Why are the judges so nasty to the contestants on televised singing contests?

To attract more viewers. In the U.K. it has calmed down a bit recently after several contestants killed themselves .

Obnoxiousness sells.Look at the money Gordon Ramsey rakes in for being a total pig.

Maybe they're professionals and they know what is proper and what needs work. 

Insulting contestants gets people to watch the show,

More people watching the show allows for a higher price for ad time during the show,

Charging more for ad time increases network profits,

Here's a rule of thumb to follow for all things of this nature,,,

If you want to know why something is don't look at the ones who suffer from it but rather at the ones that profit from it.

Those AS$HoIe are paid for