Discovery was never permanently dubbed in Turkish. You just pressed the language button and it then was in english or what ever the original soundtrack was. I seriously doubt channel D or Kanal D could afford to buy discovery but the reason its not on channel 82 anymore is digiturk couldnt come to an agreement with them (ie they didnt want to pay) so digiturk pulled the plug. The Turkish version of discovery, IZ, is just as good though.
I wish digiturk would get rid of a few other useless channels like TGRT, fashion tv and bbc prime.