I really don't think either Gabby or Julianne were popular with the fans. There was a lot of criticism, some of it here, some of it even by moi, of their hairstyle, makeup and costume choices. Neither lit the show on fire IMHO.
The Jay Leno complaint could have been a non-issue since it was edited out of the show's episode, so if the two women were that offended and complained loudly about a one-time guest judge, perhaps TPTB bristled at the two trying to run their show, and said buh-bye.
I'm delighted that when I tune in to AGT again that there will be two new judges. And hopefully no more Jay Leno.